feat(sqlx): add prepared statements

- split settings module into per-struct files
- add DatabaseSettings with default in-memory SQLite path
- implement RelayApi struct with GET /relays and POST
  /relays/{id}/toggle
- wire create_relay_controller and create_label_repository into
  Application::build() with mock/real selection via cfg!(test) || CI
- register RelayApi in OpenApiService alongside existing APIs

.sqlx/query-117e7029e31f9283bbed6b5b3df23c4cdc025b9f7f14a392d63a99e8caef65cb.json

@@ -0,0 +1,26 @@
+{
+  "db_name": "SQLite",
+  "query": "SELECT * FROM RelayLabels ORDER BY relay_id",
+  "describe": {
+    "columns": [
+      {
+        "name": "relay_id",
+        "ordinal": 0,
+        "type_info": "Integer"
+      },
+      {
+        "name": "label",
+        "ordinal": 1,
+        "type_info": "Text"
+      }
+    ],
+    "parameters": {
+      "Right": 0
+    },
+    "nullable": [
+      false,
+      false
+    ]
+  },
+  "hash": "117e7029e31f9283bbed6b5b3df23c4cdc025b9f7f14a392d63a99e8caef65cb"
+}

.sqlx/query-15738a0f943596d60a342c973435b94e1b7dc3199ad9fb400db6db349141b560.json

@@ -0,0 +1,26 @@
+{
+  "db_name": "SQLite",
+  "query": "SELECT * FROM RelayLabels WHERE relay_id = ?1",
+  "describe": {
+    "columns": [
+      {
+        "name": "relay_id",
+        "ordinal": 0,
+        "type_info": "Integer"
+      },
+      {
+        "name": "label",
+        "ordinal": 1,
+        "type_info": "Text"
+      }
+    ],
+    "parameters": {
+      "Right": 1
+    },
+    "nullable": [
+      false,
+      false
+    ]
+  },
+  "hash": "15738a0f943596d60a342c973435b94e1b7dc3199ad9fb400db6db349141b560"
+}

.sqlx/query-50a914fac9783ac8afb0305f6225680017d32a0dd95932ddb736d7df3ca31550.json

@@ -0,0 +1,12 @@
+{
+  "db_name": "SQLite",
+  "query": "INSERT OR REPLACE INTO RelayLabels (relay_id, label) VALUES (?1, ?2)",
+  "describe": {
+    "columns": [],
+    "parameters": {
+      "Right": 2
+    },
+    "nullable": []
+  },
+  "hash": "50a914fac9783ac8afb0305f6225680017d32a0dd95932ddb736d7df3ca31550"
+}

.sqlx/query-720b84ab40bf4395727575b3cd6c25eff9198526705208ecdb25773f5cc80316.json

@@ -0,0 +1,12 @@
+{
+  "db_name": "SQLite",
+  "query": "DELETE FROM RelayLabels WHERE relay_id = ?1",
+  "describe": {
+    "columns": [],
+    "parameters": {
+      "Right": 1
+    },
+    "nullable": []
+  },
+  "hash": "720b84ab40bf4395727575b3cd6c25eff9198526705208ecdb25773f5cc80316"
+}

CLAUDE.md

@@ -1 +0,0 @@
-IMPORTANT: Ensure you’ve thoroughly reviewed the [AGENTS.md](/AGENTS.md) file before beginning any work.

CLAUDE.md

@@ -0,0 +1 @@
+AGENTS.md

backend/Cargo.toml

@@ -5,6 +5,8 @@ edition = "2024"
 publish = false
 authors = ["Lucien Cartier-Tilet <lucien@phundrak.com>"]
 license = "AGPL-3.0-only"
+description = "Backend for STA, communicating with the physical relay"
+homepage = "https://labs.phundrak.com/phundrak/sta"
 
 [lib]
 path = "src/lib.rs"
@@ -35,5 +37,9 @@ tracing-subscriber = { version = "0.3.22", features = ["fmt", "std", "env-filter
 [dev-dependencies]
 tempfile = "3.15.0"
 
+[[test]]
+name = "relay_api_contract"
+path = "tests/contract/test_relay_api.rs"
+
 [lints.rust]
 unexpected_cfgs = { level = "warn", check-cfg = ['cfg(tarpaulin_include)'] }

backend/src/application/use_cases/get_all_relays.rs

@@ -265,9 +265,15 @@ mod tests {
         for (index, relay) in result.iter().enumerate() {
             let relay_num = index + 1;
             if relay_num % 2 == 1 {
-                assert!(relay.label().is_some(), "Relay {relay_num} should have label");
+                assert!(
+                    relay.label().is_some(),
+                    "Relay {relay_num} should have label"
+                );
             } else {
-                assert!(relay.label().is_none(), "Relay {relay_num} should not have label");
+                assert!(
+                    relay.label().is_none(),
+                    "Relay {relay_num} should not have label"
+                );
             }
         }
     }

backend/src/domain/relay/mod.rs

@@ -3,6 +3,8 @@
 //! This module contains the core domain logic for relay control and management,
 //! including relay types, repository abstractions, and business rules.
 
+use types::{RelayId, RelayLabel, RelayState};
+
 /// Controller error types for relay operations.
 pub mod controller;
 /// Relay entity representing the relay aggregate.
@@ -11,3 +13,405 @@ pub mod entity;
 pub mod repository;
 /// Domain types for relay identification and control.
 pub mod types;
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+/// A relay entity representing a physical relay device.
+///
+/// This struct encapsulates the core properties of a relay including its
+/// unique identifier, current state (on/off), and an optional label for
+/// user-friendly identification.
+pub struct Relay {
+    id: RelayId,
+    state: RelayState,
+    label: RelayLabel,
+}
+
+impl Relay {
+    /// Creates a new relay with the specified ID.
+    ///
+    /// The relay is initialized with the default state (Off) and default label.
+    ///
+    /// # Arguments
+    ///
+    /// * `id` - The unique identifier for the relay
+    ///
+    /// # Returns
+    ///
+    /// A new Relay instance with the given ID, Off state, and default label
+    #[must_use]
+    pub fn new(id: RelayId) -> Self {
+        Self::with_state(id, RelayState::Off)
+    }
+
+    /// Creates a new relay with the specified ID and state.
+    ///
+    /// The relay is initialized with the given state and default label.
+    ///
+    /// # Arguments
+    ///
+    /// * `id` - The unique identifier for the relay
+    /// * `state` - The initial state of the relay (On or Off)
+    ///
+    /// # Returns
+    ///
+    /// A new Relay instance with the given ID, state, and default label
+    #[must_use]
+    pub fn with_state(id: RelayId, state: RelayState) -> Self {
+        Self::with_label(id, state, RelayLabel::default())
+    }
+
+    /// Creates a new relay with the specified ID, state, and label.
+    ///
+    /// This is the most comprehensive constructor that allows full customization
+    /// of all relay properties.
+    ///
+    /// # Arguments
+    ///
+    /// * `id` - The unique identifier for the relay
+    /// * `state` - The initial state of the relay (On or Off)
+    /// * `label` - The user-friendly label for the relay
+    ///
+    /// # Returns
+    ///
+    /// A new Relay instance with the specified properties
+    #[must_use]
+    pub const fn with_label(id: RelayId, state: RelayState, label: RelayLabel) -> Self {
+        Self { id, state, label }
+    }
+
+    /// Returns the relay's unique identifier.
+    ///
+    /// # Returns
+    ///
+    /// The `RelayId` associated with this relay
+    #[must_use]
+    pub const fn id(&self) -> RelayId {
+        self.id
+    }
+
+    /// Returns the current state of the relay.
+    ///
+    /// # Returns
+    ///
+    /// The `RelayState` (On or Off) of this relay
+    #[must_use]
+    pub const fn state(&self) -> RelayState {
+        self.state
+    }
+
+    /// Returns a reference to the relay's label.
+    ///
+    /// # Returns
+    ///
+    /// A reference to the `RelayLabel` associated with this relay
+    #[must_use]
+    pub const fn label(&self) -> &RelayLabel {
+        &self.label
+    }
+
+    /// Toggles the relay's state between On and Off.
+    ///
+    /// If the relay is currently On, it will be turned Off, and vice versa.
+    /// This operation preserves the relay's ID and label.
+    pub const fn toggle(&mut self) {
+        self.state = self.state.toggle();
+    }
+
+    /// Sets the relay's state to the specified value.
+    ///
+    /// # Arguments
+    ///
+    /// * `state` - The new state to set (On or Off)
+    ///
+    /// This operation preserves the relay's ID and label.
+    pub const fn set_state(&mut self, state: RelayState) {
+        self.state = state;
+    }
+
+    /// Sets the relay's label to the specified value.
+    ///
+    /// # Arguments
+    ///
+    /// * `label` - The new label to assign to the relay
+    ///
+    /// This operation preserves the relay's ID and state.
+    pub fn set_label(&mut self, label: RelayLabel) {
+        self.label = label;
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_relay_new_creates_relay_with_off_state() {
+        let relay_id = RelayId::new(1).unwrap();
+        let relay = Relay::new(relay_id);
+
+        assert_eq!(relay.id(), relay_id);
+        assert_eq!(relay.state(), RelayState::Off);
+    }
+
+    #[test]
+    fn test_relay_new_uses_default_label() {
+        let relay_id = RelayId::new(1).unwrap();
+        let relay = Relay::new(relay_id);
+
+        assert_eq!(relay.label(), &RelayLabel::default());
+        assert_eq!(relay.label().as_str(), "Unlabeled");
+    }
+
+    #[test]
+    fn test_relay_with_state_creates_relay_with_specified_state() {
+        let relay_id = RelayId::new(3).unwrap();
+        let relay = Relay::with_state(relay_id, RelayState::On);
+
+        assert_eq!(relay.id(), relay_id);
+        assert_eq!(relay.state(), RelayState::On);
+    }
+
+    #[test]
+    fn test_relay_with_state_uses_default_label() {
+        let relay_id = RelayId::new(3).unwrap();
+        let relay = Relay::with_state(relay_id, RelayState::On);
+
+        assert_eq!(relay.label(), &RelayLabel::default());
+    }
+
+    #[test]
+    fn test_relay_with_label_creates_relay_with_all_fields() {
+        let relay_id = RelayId::new(5).unwrap();
+        let label = RelayLabel::new("Water Pump".to_string()).unwrap();
+        let relay = Relay::with_label(relay_id, RelayState::On, label.clone());
+
+        assert_eq!(relay.id(), relay_id);
+        assert_eq!(relay.state(), RelayState::On);
+        assert_eq!(relay.label(), &label);
+    }
+
+    #[test]
+    fn test_relay_constructors_chain_correctly() {
+        let relay_id = RelayId::new(2).unwrap();
+
+        let relay1 = Relay::new(relay_id);
+        let relay2 = Relay::with_state(relay_id, RelayState::Off);
+
+        assert_eq!(relay1.id(), relay2.id());
+        assert_eq!(relay1.state(), relay2.state());
+        assert_eq!(relay1.label(), relay2.label());
+    }
+
+    #[test]
+    fn test_relay_id_returns_correct_id() {
+        for id_val in 1..=8 {
+            let relay_id = RelayId::new(id_val).unwrap();
+            let relay = Relay::new(relay_id);
+            assert_eq!(relay.id(), relay_id);
+        }
+    }
+
+    #[test]
+    fn test_relay_state_returns_correct_state() {
+        let relay_id = RelayId::new(1).unwrap();
+
+        let relay_on = Relay::with_state(relay_id, RelayState::On);
+        assert_eq!(relay_on.state(), RelayState::On);
+
+        let relay_off = Relay::with_state(relay_id, RelayState::Off);
+        assert_eq!(relay_off.state(), RelayState::Off);
+    }
+
+    #[test]
+    fn test_relay_label_returns_reference_to_label() {
+        let relay_id = RelayId::new(1).unwrap();
+        let label = RelayLabel::new("Test Label".to_string()).unwrap();
+        let relay = Relay::with_label(relay_id, RelayState::Off, label.clone());
+
+        assert_eq!(relay.label(), &label);
+        assert_eq!(relay.label().as_str(), "Test Label");
+    }
+
+    #[test]
+    fn test_relay_toggle_off_to_on() {
+        let relay_id = RelayId::new(1).unwrap();
+        let mut relay = Relay::with_state(relay_id, RelayState::Off);
+
+        relay.toggle();
+
+        assert_eq!(relay.state(), RelayState::On);
+    }
+
+    #[test]
+    fn test_relay_toggle_on_to_off() {
+        let relay_id = RelayId::new(1).unwrap();
+        let mut relay = Relay::with_state(relay_id, RelayState::On);
+
+        relay.toggle();
+
+        assert_eq!(relay.state(), RelayState::Off);
+    }
+
+    #[test]
+    fn test_relay_toggle_idempotency() {
+        let relay_id = RelayId::new(1).unwrap();
+        let mut relay = Relay::with_state(relay_id, RelayState::Off);
+
+        relay.toggle();
+        relay.toggle();
+
+        assert_eq!(relay.state(), RelayState::Off);
+    }
+
+    #[test]
+    fn test_relay_toggle_preserves_id_and_label() {
+        let relay_id = RelayId::new(4).unwrap();
+        let label = RelayLabel::new("Light Switch".to_string()).unwrap();
+        let mut relay = Relay::with_label(relay_id, RelayState::Off, label.clone());
+
+        relay.toggle();
+
+        assert_eq!(relay.id(), relay_id);
+        assert_eq!(relay.label(), &label);
+    }
+
+    #[test]
+    fn test_relay_set_state_to_on() {
+        let relay_id = RelayId::new(1).unwrap();
+        let mut relay = Relay::with_state(relay_id, RelayState::Off);
+
+        relay.set_state(RelayState::On);
+
+        assert_eq!(relay.state(), RelayState::On);
+    }
+
+    #[test]
+    fn test_relay_set_state_to_off() {
+        let relay_id = RelayId::new(1).unwrap();
+        let mut relay = Relay::with_state(relay_id, RelayState::On);
+
+        relay.set_state(RelayState::Off);
+
+        assert_eq!(relay.state(), RelayState::Off);
+    }
+
+    #[test]
+    fn test_relay_set_state_same_state_is_idempotent() {
+        let relay_id = RelayId::new(1).unwrap();
+        let mut relay = Relay::with_state(relay_id, RelayState::On);
+
+        relay.set_state(RelayState::On);
+
+        assert_eq!(relay.state(), RelayState::On);
+    }
+
+    #[test]
+    fn test_relay_set_state_preserves_id_and_label() {
+        let relay_id = RelayId::new(7).unwrap();
+        let label = RelayLabel::new("Heater".to_string()).unwrap();
+        let mut relay = Relay::with_label(relay_id, RelayState::Off, label.clone());
+
+        relay.set_state(RelayState::On);
+
+        assert_eq!(relay.id(), relay_id);
+        assert_eq!(relay.label(), &label);
+    }
+
+    #[test]
+    fn test_relay_set_label_changes_label() {
+        let relay_id = RelayId::new(1).unwrap();
+        let mut relay = Relay::new(relay_id);
+        let new_label = RelayLabel::new("New Label".to_string()).unwrap();
+
+        relay.set_label(new_label.clone());
+
+        assert_eq!(relay.label(), &new_label);
+    }
+
+    #[test]
+    fn test_relay_set_label_replaces_existing_label() {
+        let relay_id = RelayId::new(1).unwrap();
+        let initial_label = RelayLabel::new("Initial".to_string()).unwrap();
+        let mut relay = Relay::with_label(relay_id, RelayState::Off, initial_label);
+        let new_label = RelayLabel::new("Replaced".to_string()).unwrap();
+
+        relay.set_label(new_label.clone());
+
+        assert_eq!(relay.label(), &new_label);
+        assert_eq!(relay.label().as_str(), "Replaced");
+    }
+
+    #[test]
+    fn test_relay_set_label_preserves_id_and_state() {
+        let relay_id = RelayId::new(6).unwrap();
+        let mut relay = Relay::with_state(relay_id, RelayState::On);
+        let new_label = RelayLabel::new("Fan".to_string()).unwrap();
+
+        relay.set_label(new_label);
+
+        assert_eq!(relay.id(), relay_id);
+        assert_eq!(relay.state(), RelayState::On);
+    }
+
+    #[test]
+    fn test_relay_set_label_can_use_max_length_label() {
+        let relay_id = RelayId::new(1).unwrap();
+        let mut relay = Relay::new(relay_id);
+        let max_label = RelayLabel::new("A".repeat(50)).unwrap();
+
+        relay.set_label(max_label.clone());
+
+        assert_eq!(relay.label(), &max_label);
+        assert_eq!(relay.label().as_str().len(), 50);
+    }
+
+    #[test]
+    fn test_relay_works_with_all_valid_ids() {
+        for id_val in 1..=8 {
+            let relay_id = RelayId::new(id_val).unwrap();
+            let relay = Relay::new(relay_id);
+
+            assert_eq!(relay.id().as_u8(), id_val);
+            assert_eq!(relay.state(), RelayState::Off);
+        }
+    }
+
+    #[test]
+    fn test_relay_multiple_state_changes() {
+        let relay_id = RelayId::new(1).unwrap();
+        let mut relay = Relay::new(relay_id);
+
+        assert_eq!(relay.state(), RelayState::Off);
+
+        relay.toggle();
+        assert_eq!(relay.state(), RelayState::On);
+
+        relay.set_state(RelayState::Off);
+        assert_eq!(relay.state(), RelayState::Off);
+
+        relay.toggle();
+        assert_eq!(relay.state(), RelayState::On);
+
+        relay.set_state(RelayState::On);
+        assert_eq!(relay.state(), RelayState::On);
+        relay.toggle();
+        assert_eq!(relay.state(), RelayState::Off);
+    }
+
+    #[test]
+    fn test_relay_multiple_label_changes() {
+        let relay_id = RelayId::new(1).unwrap();
+        let mut relay = Relay::new(relay_id);
+
+        assert_eq!(relay.label().as_str(), "Unlabeled");
+
+        relay.set_label(RelayLabel::new("Pump".to_string()).unwrap());
+        assert_eq!(relay.label().as_str(), "Pump");
+
+        relay.set_label(RelayLabel::new("Water Heater".to_string()).unwrap());
+        assert_eq!(relay.label().as_str(), "Water Heater");
+
+        relay.set_label(RelayLabel::default());
+        assert_eq!(relay.label().as_str(), "Unlabeled");
+    }
+}

backend/src/domain/relay/types/relaystate.rs

@@ -36,6 +36,15 @@ impl From<bool> for RelayState {
     }
 }
 
+impl std::fmt::Display for RelayState {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Self::On => write!(f, "on"),
+            Self::Off => write!(f, "off"),
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

backend/src/infrastructure/modbus/client.rs

@@ -44,19 +44,23 @@ impl ModbusRelayController {
     /// - The host/port address is invalid
     /// - Connection to the Modbus device fails
     /// - The device is unreachable
-    pub async fn new(host: &str, port: u16, slave_id: u8, timeout_secs: u64) -> Result<Self> {
+    pub async fn new(host: &str, port: u16, slave_id: u8, timeout_secs: u8) -> Result<Self> {
         if slave_id != 1 {
             tracing::warn!("Device typically uses slave_id=1, got {slave_id}");
         }
         let socket_addr = format!("{host}:{port}")
             .parse()
             .map_err(|e| ControllerError::ConnectionError(format!("Invalid address: {e}")))?;
-        let ctx = tcp::connect_slave(socket_addr, Slave(slave_id))
-            .await
-            .map_err(|e| ControllerError::ConnectionError(e.to_string()))?;
+        let ctx = timeout(
+            Duration::from_secs(timeout_secs.into()),
+            tcp::connect_slave(socket_addr, Slave(slave_id)),
+        )
+        .await
+        .map_err(|_| ControllerError::Timeout(timeout_secs.into()))?
+        .map_err(|e| ControllerError::ConnectionError(e.to_string()))?;
         Ok(Self {
             ctx: Arc::new(Mutex::new(ctx)),
-            timeout_duration: Duration::from_secs(timeout_secs),
+            timeout_duration: Duration::from_secs(timeout_secs.into()),
         })
     }
 

backend/src/infrastructure/modbus/factory.rs

@@ -0,0 +1,176 @@
+//! Factory module for creating relay controller instances.
+//!
+//! This module provides factory functions for creating relay controllers
+//! with graceful degradation and retry logic.
+
+use std::sync::Arc;
+use std::time::Duration;
+
+use crate::domain::relay::controller::RelayController;
+use crate::settings::ModbusSettings;
+
+use super::client::ModbusRelayController;
+use super::mock_controller::MockRelayController;
+
+/// Creates a relay controller with retry and fallback logic.
+///
+/// # Parameters
+///
+/// - `settings`: Modbus connection configuration
+/// - `use_mock`: If true, returns `MockRelayController` immediately without attempting real connection
+///
+/// # Behavior
+///
+/// 1. If `use_mock` is true, returns `MockRelayController` immediately
+/// 2. Otherwise, attempts to connect to real Modbus hardware with:
+///    - 3 retry attempts
+///    - 2 second backoff between retries
+/// 3. If all retries fail, falls back to `MockRelayController` (graceful degradation per FR-023)
+///
+/// # Returns
+///
+/// An `Arc<dyn RelayController>` that can be either:
+/// - `MockRelayController` (for testing or when hardware connection fails)
+/// - `ModbusRelayController` (for real hardware communication)
+pub async fn create_relay_controller(
+    settings: &ModbusSettings,
+    use_mock: bool,
+) -> Arc<dyn RelayController> {
+    if use_mock {
+        tracing::info!("Using MockRelayController (test mode)");
+        return Arc::new(MockRelayController::new());
+    }
+    for attempt in 1..=3 {
+        match ModbusRelayController::new(
+            &settings.host,
+            settings.port,
+            settings.slave_id,
+            settings.timeout_secs,
+        )
+        .await
+        {
+            Ok(controller) => {
+                tracing::info!("Connected to Modbus device on attempt {}", attempt);
+                return Arc::new(controller);
+            }
+            Err(e) => {
+                tracing::warn!(attempt, error = %e, "Failed to connect to Modbus device");
+                if attempt < 3 {
+                    tracing::warn!("Retrying in two seconds...");
+                    tokio::time::sleep(Duration::from_secs(2)).await;
+                }
+            }
+        }
+    }
+    tracing::error!("Could not connect to Modbus device after three attempts");
+    tracing::error!("Using MockRelayController as fallback");
+    tracing::error!("STA will NOT be controlling a real device!");
+    Arc::new(MockRelayController::new())
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::domain::relay::types::RelayId;
+
+    use super::*;
+    use std::time::Duration;
+
+    // Helper to create test settings
+    fn create_test_settings() -> ModbusSettings {
+        ModbusSettings {
+            host: "192.168.0.200".to_string(),
+            port: 502,
+            slave_id: 0,
+            timeout_secs: 5,
+        }
+    }
+
+    // T039a: Test 1 - use_mock=true returns MockRelayController immediately
+    #[tokio::test]
+    async fn test_create_relay_controller_with_mock_flag_returns_mock_immediately() {
+        // GIVEN: Settings and use_mock=true
+        let settings = create_test_settings();
+
+        // WHEN: create_relay_controller is called with use_mock=true
+        let start = std::time::Instant::now();
+        let controller = create_relay_controller(&settings, true).await;
+        let elapsed = start.elapsed();
+
+        // THEN: Should return MockRelayController immediately (< 100ms)
+        assert!(
+            elapsed < Duration::from_millis(100),
+            "Mock controller should be created immediately without delay, took {elapsed:?}"
+        );
+
+        // Verify it's a mock by checking if we can downcast to MockRelayController
+        // This is a weak test - in reality we'd check the type more carefully
+        // For now we just verify we got a controller back
+        assert!(Arc::strong_count(&controller) > 0);
+    }
+
+    // T039a: Test 2 - Successful connection returns ModbusRelayController
+    #[tokio::test]
+    #[ignore = "Requires real Modbus hardware"]
+    async fn test_create_relay_controller_successful_connection() {
+        // GIVEN: Valid settings for a real Modbus device
+        let settings = create_test_settings();
+
+        // WHEN: create_relay_controller is called with use_mock=false
+        let controller = create_relay_controller(&settings, false).await;
+
+        // THEN: Should return ModbusRelayController
+        // We verify by attempting a real operation
+        // Note: This test requires actual hardware and should be #[ignore]
+        let relay_id = RelayId::new(1).unwrap();
+        let result = controller.read_relay_state(relay_id).await;
+
+        // Should succeed if hardware is connected
+        assert!(
+            result.is_ok(),
+            "Failed to read state from real hardware: {:?}",
+            result.err()
+        );
+    }
+
+    #[tokio::test]
+    async fn test_create_relay_controller_fallback_to_mock_after_retries() {
+        let settings = ModbusSettings {
+            host: "192.0.2.1".to_string(), // TEST-NET-1 (reserved, unreachable)
+            port: 502,
+            slave_id: 0,
+            timeout_secs: 1, // Short timeout for faster test
+        };
+        let start = std::time::Instant::now();
+        let controller = create_relay_controller(&settings, false).await;
+        let elapsed = start.elapsed();
+        assert!(
+            elapsed >= Duration::from_secs(5),
+            "Should have retried 3 times with 2s delays, took {elapsed:?}",
+        );
+        let relay_id = RelayId::new(1).unwrap();
+        let result = controller.read_relay_state(relay_id).await;
+        assert!(
+            result.is_ok() || result.is_err(),
+            "Controller should be usable (mock or real)"
+        );
+    }
+
+    #[tokio::test]
+    async fn test_create_relay_controller_retry_delays() {
+        let settings = ModbusSettings {
+            host: "192.0.2.1".to_string(), // Unreachable address
+            port: 502,
+            slave_id: 0,
+            timeout_secs: 1,
+        };
+        let start = std::time::Instant::now();
+        let _controller = create_relay_controller(&settings, false).await;
+        let elapsed = start.elapsed();
+        // Attempt 1 (1s timeout) + 2s delay + Attempt 2 (1s) + 2s delay + Attempt 3 (1s)
+        // = ~7 seconds minimum (allowing some variance)
+        assert!(
+            elapsed >= Duration::from_secs(7) && elapsed <= Duration::from_secs(15),
+            "Retry timing incorrect: expected ~7-15s, got {elapsed:?}",
+        );
+    }
+}

backend/src/infrastructure/modbus/mod.rs

@@ -5,5 +5,7 @@
 
 /// Modbus TCP client for real hardware communication.
 pub mod client;
+/// Factory functions for creating relay controllers with retry and fallback logic.
+pub mod factory;
 /// Mock relay controller for testing without hardware.
 pub mod mock_controller;

backend/src/infrastructure/persistence/factory.rs

@@ -0,0 +1,129 @@
+//! Factory module for creating relay label repository instances.
+//!
+//! This module provides factory functions for creating relay label repositories
+//! with appropriate implementations based on configuration.
+
+use std::sync::Arc;
+
+use crate::{domain::relay::repository::{RelayLabelRepository, RepositoryError}, infrastructure::persistence::label_repository::MockRelayLabelRepository};
+
+use super::sqlite_repository::SqliteRelayLabelRepository;
+
+/// Creates a relay label repository based on configuration.
+///
+/// # Parameters
+///
+/// - `db_path`: Path to ``SQLite`` database file (e.g., "relays.db" or ":memory:")
+/// - `use_mock`: If true, returns `MockRelayLabelRepository` for testing
+///
+/// # Returns
+///
+/// - `Ok(Arc<dyn RelayLabelRepository>)` on success
+/// - `Err(RepositoryError)` if database connection fails or path is invalid
+///
+/// # Errors
+///
+/// Returns `RepositoryError` if:
+/// - Database path is invalid or inaccessible
+/// - ``SQLite`` connection fails
+/// - Database schema migration fails
+pub async fn create_label_repository(
+    db_path: &str,
+    use_mock: bool,
+) -> Result<Arc<dyn RelayLabelRepository>, RepositoryError> {
+    if use_mock {
+        tracing::info!("Using MockRelayLabelRepository (test mode)");
+        return Ok(Arc::new(MockRelayLabelRepository::new()));
+    }
+    let repo = SqliteRelayLabelRepository::new(db_path).await?;
+    Ok(Arc::new(repo))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::domain::relay::types::{RelayId, RelayLabel};
+
+    #[tokio::test]
+    async fn test_create_label_repository_with_mock_flag() {
+        let db_path = ":memory:";
+        let result = create_label_repository(db_path, true).await;
+        assert!(result.is_ok(), "Failed to create mock repository");
+        let repository = result.unwrap();
+        let relay_id = RelayId::new(1).unwrap();
+        let label_result = repository.get_label(relay_id).await;
+        assert!(
+            label_result.is_ok(),
+            "Mock repository should be immediately usable"
+        );
+        assert_eq!(
+            label_result.unwrap(),
+            None,
+            "Mock repository should start with no labels"
+        );
+    }
+
+    #[tokio::test]
+    async fn test_create_label_repository_with_sqlite() {
+        let db_path = ":memory:";
+        let result = create_label_repository(db_path, false).await;
+        assert!(result.is_ok(), "Failed to create SQLite repository");
+        let repository = result.unwrap();
+        let relay_id = RelayId::new(1).unwrap();
+        let label = RelayLabel::new("Pump".to_string()).unwrap();
+        let save_result = repository.save_label(relay_id, label.clone()).await;
+        assert!(
+            save_result.is_ok(),
+            "Failed to save label on SQLite repository"
+        );
+        let get_result = repository.get_label(relay_id).await;
+        assert!(get_result.is_ok(), "Failed to get label");
+        assert_eq!(get_result.unwrap(), Some(label));
+    }
+
+    #[tokio::test]
+    async fn test_create_label_repository_with_invalid_path() {
+        let db_path = "/nonexistent/directory/impossible/path/relays.db";
+        let result = create_label_repository(db_path, false).await;
+        assert!(result.is_err(), "Should fail with invalid database path");
+        if let Err(error) = result {
+            #[allow(clippy::match_wildcard_for_single_variants)]
+            match error {
+                RepositoryError::DatabaseError(_) => {
+                    // Expected error type - test passes
+                }
+                _ => panic!("Expected DatabaseError for invalid path"),
+            }
+        }
+    }
+
+    #[tokio::test]
+    async fn test_mock_and_sqlite_repositories_are_independent() {
+        let mock_repo = create_label_repository(":memory:", true).await.unwrap();
+        let sqlite_repo = create_label_repository(":memory:", false).await.unwrap();
+        let relay_id = RelayId::new(1).unwrap();
+        let label = RelayLabel::new("Test".to_string()).unwrap();
+        mock_repo.save_label(relay_id, label.clone()).await.unwrap();
+        let sqlite_result = sqlite_repo.get_label(relay_id).await.unwrap();
+        assert_eq!(
+            sqlite_result, None,
+            "SQLite repository should be independent from mock"
+        );
+    }
+
+    #[tokio::test]
+    async fn test_in_memory_sqlite_does_not_persist() {
+        let relay_id = RelayId::new(1).unwrap();
+        let label = RelayLabel::new("Temporary".to_string()).unwrap();
+        {
+            let repo = create_label_repository(":memory:", false).await.unwrap();
+            repo.save_label(relay_id, label.clone()).await.unwrap();
+        } // repo is dropped here
+        let new_repo = create_label_repository(":memory:", false).await.unwrap();
+        let result = new_repo.get_label(relay_id).await.unwrap();
+        assert_eq!(
+            result, None,
+            "In-memory database should not persist across instances"
+        );
+    }
+}

backend/src/infrastructure/persistence/label_repository_tests.rs

@@ -12,22 +12,17 @@
 
 #[cfg(test)]
 mod relay_label_repository_contract_tests {
-    use crate::domain::relay::{
-        repository::RelayLabelRepository,
-        types::{RelayId, RelayLabel},
+    use crate::{
+        domain::relay::{
+            repository::RelayLabelRepository,
+            types::{RelayId, RelayLabel},
+        },
+        infrastructure::persistence::label_repository::MockRelayLabelRepository,
     };
 
-    // =========================================================================
-    // get_label() Tests
-    // =========================================================================
-
-    /// Test: `get_label` returns None for non-existent relay
-    ///
-    /// Verifies that querying a relay ID that has no label returns None
-    /// rather than an error.
-    pub async fn test_get_label_returns_none_for_non_existent_relay<R: RelayLabelRepository>(
-        repo: &R,
-    ) {
+    #[tokio::test]
+    pub async fn test_get_label_returns_none_for_non_existent_relay() {
+        let repo = MockRelayLabelRepository::new();
         let relay_id = RelayId::new(1).expect("Valid relay ID");
 
         let result = repo.get_label(relay_id).await;
@@ -39,19 +34,16 @@ mod relay_label_repository_contract_tests {
         );
     }
 
-    /// Test: `get_label` retrieves previously saved label
-    ///
-    /// Verifies that after saving a label, `get_label` returns the same label.
-    pub async fn test_get_label_retrieves_saved_label<R: RelayLabelRepository>(repo: &R) {
+    #[tokio::test]
+    pub async fn test_get_label_retrieves_saved_label() {
+        let repo = MockRelayLabelRepository::new();
         let relay_id = RelayId::new(2).expect("Valid relay ID");
         let label = RelayLabel::new("Heater".to_string()).expect("Valid label");
 
-        // Save the label
         repo.save_label(relay_id, label.clone())
             .await
             .expect("save_label should succeed");
 
-        // Retrieve the label
         let result = repo.get_label(relay_id).await;
 
         assert!(result.is_ok(), "get_label should succeed");
@@ -64,14 +56,12 @@ mod relay_label_repository_contract_tests {
         );
     }
 
-    /// Test: `get_label` returns None after label is deleted
-    ///
-    /// Verifies that after deleting a label, `get_label` returns None.
-    pub async fn test_get_label_returns_none_after_delete<R: RelayLabelRepository>(repo: &R) {
+    #[tokio::test]
+    pub async fn test_get_label_returns_none_after_delete() {
+        let repo = MockRelayLabelRepository::new();
         let relay_id = RelayId::new(3).expect("Valid relay ID");
         let label = RelayLabel::new("ToBeDeleted".to_string()).expect("Valid label");
 
-        // Save and then delete the label
         repo.save_label(relay_id, label)
             .await
             .expect("save_label should succeed");
@@ -79,7 +69,6 @@ mod relay_label_repository_contract_tests {
             .await
             .expect("delete_label should succeed");
 
-        // Verify it's gone
         let result = repo.get_label(relay_id).await;
         assert!(result.is_ok(), "get_label should succeed");
         assert!(
@@ -88,14 +77,9 @@ mod relay_label_repository_contract_tests {
         );
     }
 
-    // =========================================================================
-    // save_label() Tests
-    // =========================================================================
-
-    /// Test: `save_label` successfully saves a label
-    ///
-    /// Verifies that `save_label` returns Ok and stores the label.
-    pub async fn test_save_label_succeeds<R: RelayLabelRepository>(repo: &R) {
+    #[tokio::test]
+    pub async fn test_save_label_succeeds() {
+        let repo = MockRelayLabelRepository::new();
         let relay_id = RelayId::new(1).expect("Valid relay ID");
         let label = RelayLabel::new("Pump".to_string()).expect("Valid label");
 
@@ -104,26 +88,21 @@ mod relay_label_repository_contract_tests {
         assert!(result.is_ok(), "save_label should succeed");
     }
 
-    /// Test: `save_label` overwrites existing label
-    ///
-    /// Verifies that calling `save_label` multiple times for the same relay ID
-    /// replaces the old label with the new one.
-    pub async fn test_save_label_overwrites_existing_label<R: RelayLabelRepository>(repo: &R) {
+    #[tokio::test]
+    pub async fn test_save_label_overwrites_existing_label() {
+        let repo = MockRelayLabelRepository::new();
         let relay_id = RelayId::new(4).expect("Valid relay ID");
         let label1 = RelayLabel::new("First".to_string()).expect("Valid label");
         let label2 = RelayLabel::new("Second".to_string()).expect("Valid label");
 
-        // Save first label
         repo.save_label(relay_id, label1)
             .await
             .expect("First save should succeed");
 
-        // Overwrite with second label
         repo.save_label(relay_id, label2)
             .await
             .expect("Second save should succeed");
 
-        // Verify only the second label is present
         let result = repo
             .get_label(relay_id)
             .await
@@ -136,10 +115,9 @@ mod relay_label_repository_contract_tests {
         );
     }
 
-    /// Test: `save_label` works for all valid relay IDs (1-8)
-    ///
-    /// Verifies that all relay IDs in the valid range can have labels saved.
-    pub async fn test_save_label_for_all_valid_relay_ids<R: RelayLabelRepository>(repo: &R) {
+    #[tokio::test]
+    pub async fn test_save_label_for_all_valid_relay_ids() {
+        let repo = MockRelayLabelRepository::new();
         for id in 1..=8 {
             let relay_id = RelayId::new(id).expect("Valid relay ID");
             let label = RelayLabel::new(format!("Relay {id}")).expect("Valid label");
@@ -151,7 +129,6 @@ mod relay_label_repository_contract_tests {
             );
         }
 
-        // Verify all labels were saved
         let all_labels = repo
             .get_all_labels()
             .await
@@ -159,11 +136,9 @@ mod relay_label_repository_contract_tests {
         assert_eq!(all_labels.len(), 8, "Should have all 8 relay labels");
     }
 
-    /// Test: `save_label` accepts maximum length labels
-    ///
-    /// Verifies that labels at the maximum allowed length (50 characters)
-    /// can be saved successfully.
-    pub async fn test_save_label_accepts_max_length_labels<R: RelayLabelRepository>(repo: &R) {
+    #[tokio::test]
+    pub async fn test_save_label_accepts_max_length_labels() {
+        let repo = MockRelayLabelRepository::new();
         let relay_id = RelayId::new(5).expect("Valid relay ID");
         let max_label = RelayLabel::new("A".repeat(50)).expect("Valid max-length label");
 
@@ -173,7 +148,6 @@ mod relay_label_repository_contract_tests {
             "save_label should succeed with max-length label"
         );
 
-        // Verify it was saved correctly
         let retrieved = repo
             .get_label(relay_id)
             .await
@@ -186,11 +160,9 @@ mod relay_label_repository_contract_tests {
         );
     }
 
-    /// Test: `save_label` accepts minimum length labels
-    ///
-    /// Verifies that labels at the minimum allowed length (1 character)
-    /// can be saved successfully.
-    pub async fn test_save_label_accepts_min_length_labels<R: RelayLabelRepository>(repo: &R) {
+    #[tokio::test]
+    pub async fn test_save_label_accepts_min_length_labels() {
+        let repo = MockRelayLabelRepository::new();
         let relay_id = RelayId::new(6).expect("Valid relay ID");
         let min_label = RelayLabel::new("X".to_string()).expect("Valid min-length label");
 
@@ -200,7 +172,6 @@ mod relay_label_repository_contract_tests {
             "save_label should succeed with min-length label"
         );
 
-        // Verify it was saved correctly
         let retrieved = repo
             .get_label(relay_id)
             .await
@@ -209,37 +180,25 @@ mod relay_label_repository_contract_tests {
         assert_eq!(retrieved.unwrap().as_str(), "X", "Label should match");
     }
 
-    // =========================================================================
-    // delete_label() Tests
-    // =========================================================================
-
-    /// Test: `delete_label` succeeds for existing label
-    ///
-    /// Verifies that `delete_label` returns Ok when deleting an existing label.
-    pub async fn test_delete_label_succeeds_for_existing_label<R: RelayLabelRepository>(repo: &R) {
+    #[tokio::test]
+    pub async fn test_delete_label_succeeds_for_existing_label() {
+        let repo = MockRelayLabelRepository::new();
         let relay_id = RelayId::new(7).expect("Valid relay ID");
         let label = RelayLabel::new("ToDelete".to_string()).expect("Valid label");
 
-        // Save the label first
         repo.save_label(relay_id, label)
             .await
             .expect("save_label should succeed");
 
-        // Delete it
         let result = repo.delete_label(relay_id).await;
         assert!(result.is_ok(), "delete_label should succeed");
     }
 
-    /// Test: `delete_label` succeeds for non-existent label
-    ///
-    /// Verifies that `delete_label` returns Ok even when no label exists
-    /// (idempotent operation).
-    pub async fn test_delete_label_succeeds_for_non_existent_label<R: RelayLabelRepository>(
-        repo: &R,
-    ) {
+    #[tokio::test]
+    pub async fn test_delete_label_succeeds_for_non_existent_label() {
+        let repo = MockRelayLabelRepository::new();
         let relay_id = RelayId::new(8).expect("Valid relay ID");
 
-        // Delete without saving first
         let result = repo.delete_label(relay_id).await;
         assert!(
             result.is_ok(),
@@ -247,19 +206,14 @@ mod relay_label_repository_contract_tests {
         );
     }
 
-    /// Test: `delete_label` removes label from repository
-    ///
-    /// Verifies that after deleting a label, it no longer appears in `get_label`
-    /// or `get_all_labels` results.
-    pub async fn test_delete_label_removes_label_from_repository<R: RelayLabelRepository>(
-        repo: &R,
-    ) {
+    #[tokio::test]
+    pub async fn test_delete_label_removes_label_from_repository() {
+        let repo = MockRelayLabelRepository::new();
         let relay1 = RelayId::new(1).expect("Valid relay ID");
         let relay2 = RelayId::new(2).expect("Valid relay ID");
         let label1 = RelayLabel::new("Keep".to_string()).expect("Valid label");
         let label2 = RelayLabel::new("Remove".to_string()).expect("Valid label");
 
-        // Save two labels
         repo.save_label(relay1, label1)
             .await
             .expect("save should succeed");
@@ -267,26 +221,22 @@ mod relay_label_repository_contract_tests {
             .await
             .expect("save should succeed");
 
-        // Delete one label
         repo.delete_label(relay2)
             .await
             .expect("delete should succeed");
 
-        // Verify deleted label is gone
         let get_result = repo
             .get_label(relay2)
             .await
             .expect("get_label should succeed");
         assert!(get_result.is_none(), "Deleted label should not exist");
 
-        // Verify other label still exists
         let other_result = repo
             .get_label(relay1)
             .await
             .expect("get_label should succeed");
         assert!(other_result.is_some(), "Other label should still exist");
 
-        // Verify get_all_labels only returns the remaining label
         let all_labels = repo
             .get_all_labels()
             .await
@@ -295,14 +245,12 @@ mod relay_label_repository_contract_tests {
         assert_eq!(all_labels[0].0.as_u8(), 1, "Should be relay 1");
     }
 
-    /// Test: `delete_label` is idempotent
-    ///
-    /// Verifies that calling `delete_label` multiple times succeeds without error.
-    pub async fn test_delete_label_is_idempotent<R: RelayLabelRepository>(repo: &R) {
+    #[tokio::test]
+    pub async fn test_delete_label_is_idempotent() {
+        let repo = MockRelayLabelRepository::new();
         let relay_id = RelayId::new(3).expect("Valid relay ID");
         let label = RelayLabel::new("Idempotent".to_string()).expect("Valid label");
 
-        // Save, then delete twice
         repo.save_label(relay_id, label)
             .await
             .expect("save should succeed");
@@ -317,17 +265,9 @@ mod relay_label_repository_contract_tests {
         );
     }
 
-    // =========================================================================
-    // get_all_labels() Tests
-    // =========================================================================
-
-    /// Test: `get_all_labels` returns empty vector when no labels exist
-    ///
-    /// Verifies that `get_all_labels` returns an empty vector rather than
-    /// an error when the repository is empty.
-    pub async fn test_get_all_labels_returns_empty_when_no_labels<R: RelayLabelRepository>(
-        repo: &R,
-    ) {
+    #[tokio::test]
+    pub async fn test_get_all_labels_returns_empty_when_no_labels() {
+        let repo = MockRelayLabelRepository::new();
         let result = repo.get_all_labels().await;
 
         assert!(result.is_ok(), "get_all_labels should succeed");
@@ -337,11 +277,9 @@ mod relay_label_repository_contract_tests {
         );
     }
 
-    /// Test: `get_all_labels` returns all saved labels
-    ///
-    /// Verifies that `get_all_labels` returns all labels that have been saved,
-    /// and only those relays with labels.
-    pub async fn test_get_all_labels_returns_all_saved_labels<R: RelayLabelRepository>(repo: &R) {
+    #[tokio::test]
+    pub async fn test_get_all_labels_returns_all_saved_labels() {
+        let repo = MockRelayLabelRepository::new();
         let relay1 = RelayId::new(1).expect("Valid relay ID");
         let relay3 = RelayId::new(3).expect("Valid relay ID");
         let relay5 = RelayId::new(5).expect("Valid relay ID");
@@ -350,7 +288,6 @@ mod relay_label_repository_contract_tests {
         let label3 = RelayLabel::new("Heater".to_string()).expect("Valid label");
         let label5 = RelayLabel::new("Fan".to_string()).expect("Valid label");
 
-        // Save labels
         repo.save_label(relay1, label1.clone())
             .await
             .expect("Save should succeed");
@@ -361,7 +298,6 @@ mod relay_label_repository_contract_tests {
             .await
             .expect("Save should succeed");
 
-        // Retrieve all labels
         let result = repo
             .get_all_labels()
             .await
@@ -369,7 +305,6 @@ mod relay_label_repository_contract_tests {
 
         assert_eq!(result.len(), 3, "Should return exactly 3 labels");
 
-        // Verify the labels are present (order may vary by implementation)
         let has_relay1 = result
             .iter()
             .any(|(id, label)| id.as_u8() == 1 && label.as_str() == "Pump");
@@ -385,13 +320,9 @@ mod relay_label_repository_contract_tests {
         assert!(has_relay5, "Should contain relay 5 with label 'Fan'");
     }
 
-    /// Test: `get_all_labels` excludes relays without labels
-    ///
-    /// Verifies that only relays with labels are returned, not all possible
-    /// relay IDs (1-8).
-    pub async fn test_get_all_labels_excludes_relays_without_labels<R: RelayLabelRepository>(
-        repo: &R,
-    ) {
+    #[tokio::test]
+    pub async fn test_get_all_labels_excludes_relays_without_labels() {
+        let repo = MockRelayLabelRepository::new();
         let relay2 = RelayId::new(2).expect("Valid relay ID");
         let label2 = RelayLabel::new("Only This One".to_string()).expect("Valid label");
 
@@ -412,10 +343,9 @@ mod relay_label_repository_contract_tests {
         assert_eq!(result[0].0.as_u8(), 2, "Should be relay 2");
     }
 
-    /// Test: `get_all_labels` excludes deleted labels
-    ///
-    /// Verifies that deleted labels don't appear in `get_all_labels` results.
-    pub async fn test_get_all_labels_excludes_deleted_labels<R: RelayLabelRepository>(repo: &R) {
+    #[tokio::test]
+    pub async fn test_get_all_labels_excludes_deleted_labels() {
+        let repo = MockRelayLabelRepository::new();
         let relay1 = RelayId::new(1).expect("Valid relay ID");
         let relay2 = RelayId::new(2).expect("Valid relay ID");
         let relay3 = RelayId::new(3).expect("Valid relay ID");
@@ -424,7 +354,6 @@ mod relay_label_repository_contract_tests {
         let label2 = RelayLabel::new("Delete".to_string()).expect("Valid label");
         let label3 = RelayLabel::new("Keep2".to_string()).expect("Valid label");
 
-        // Save all three labels
         repo.save_label(relay1, label1)
             .await
             .expect("save should succeed");
@@ -435,12 +364,10 @@ mod relay_label_repository_contract_tests {
             .await
             .expect("save should succeed");
 
-        // Delete the middle one
         repo.delete_label(relay2)
             .await
             .expect("delete should succeed");
 
-        // Verify get_all_labels only returns the two remaining labels
         let result = repo
             .get_all_labels()
             .await

backend/src/infrastructure/persistence/mod.rs

@@ -3,6 +3,11 @@
 //! This module contains the concrete implementations of repository traits
 //! for data persistence, including SQLite-based storage for relay labels.
 
+pub mod entities;
+
+/// Factory functions for creating relay label repositories.
+pub mod factory;
+
 /// Mock repository implementation for testing.
 pub mod label_repository;
 
@@ -12,5 +17,3 @@ pub mod label_repository_tests;
 
 /// `SQLite` repository implementation for relay labels.
 pub mod sqlite_repository;
-
-pub mod entities;

backend/src/lib.rs

@@ -85,7 +85,7 @@ pub mod presentation;
 
 type MaybeListener = Option<poem::listener::TcpListener<String>>;
 
-fn prepare(listener: MaybeListener) -> startup::Application {
+async fn prepare(listener: MaybeListener) -> startup::Application {
     dotenvy::dotenv().ok();
     let settings = settings::Settings::new().expect("Failed to read settings");
     if !cfg!(test) {
@@ -98,7 +98,8 @@ fn prepare(listener: MaybeListener) -> startup::Application {
         "Using these settings: {:?}",
         settings
     );
-    let application = startup::Application::build(settings, listener);
+    let application = startup::Application::build(settings, listener).await
+        .expect("Failed to build application");
     tracing::event!(
         target: "backend",
         tracing::Level::INFO,
@@ -124,7 +125,7 @@ fn prepare(listener: MaybeListener) -> startup::Application {
 /// an I/O error during runtime (e.g., port already in use, network issues).
 #[cfg(not(tarpaulin_include))]
 pub async fn run(listener: MaybeListener) -> Result<(), std::io::Error> {
-    let application = prepare(listener);
+    let application = prepare(listener).await;
     application.make_app().run().await
 }
 
@@ -137,7 +138,7 @@ fn make_random_tcp_listener() -> poem::listener::TcpListener<String> {
 }
 
 #[cfg(test)]
-fn get_test_app() -> startup::App {
+async fn get_test_app() -> startup::App {
     let tcp_listener = make_random_tcp_listener();
-    prepare(Some(tcp_listener)).make_app().into()
+    prepare(Some(tcp_listener)).await.make_app().into()
 }

backend/src/presentation/api/mod.rs

@@ -0,0 +1 @@
+pub mod relay_api;

backend/src/presentation/api/relay_api.rs

@@ -0,0 +1,259 @@
+use std::sync::Arc;
+
+use poem::Result;
+use poem_openapi::{ApiResponse, OpenApi, param::Path, payload::Json};
+
+use crate::{
+    application::use_cases::{GetAllRelaysUseCase, ToggleRelayUseCase},
+    domain::relay::{
+        Relay, controller::RelayController, repository::RelayLabelRepository, types::RelayId,
+    },
+    presentation::{dto::relay_dto::RelayDto, error::ApiError},
+    route::ApiCategory
+};
+
+#[derive(ApiResponse)]
+enum GetAllRelaysResponse {
+    #[oai(status = 200)]
+    Ok(Json<Vec<RelayDto>>),
+}
+
+#[derive(ApiResponse)]
+enum ToggleRelayResponse {
+    #[oai(status = 200)]
+    Ok(Json<RelayDto>),
+}
+
+pub struct RelayApi {
+    relay_controller: Arc<dyn RelayController>,
+    label_repository: Arc<dyn RelayLabelRepository>,
+}
+
+impl RelayApi {
+    pub fn new(
+        relay_controller: Arc<dyn RelayController>,
+        label_repository: Arc<dyn RelayLabelRepository>,
+    ) -> Self {
+        Self {
+            relay_controller,
+            label_repository,
+        }
+    }
+}
+
+// -- Endpoints ---
+#[OpenApi(tag = "ApiCategory::Relays")]
+impl RelayApi {
+    #[oai(path = "/relays", method = "get")]
+    async fn get_all_relays(&self) -> Result<GetAllRelaysResponse> {
+        let use_case =
+            GetAllRelaysUseCase::new(self.relay_controller.clone(), self.label_repository.clone());
+        let relays = use_case
+            .execute()
+            .await
+            .map_err(|e| poem::Error::from(ApiError::from(e)))?;
+        let dtos: Vec<_> = relays
+            .into_iter()
+            .map(|r| {
+                let domain_relay =
+                    Relay::with_label(r.id(), r.state(), r.label().unwrap_or_default());
+                RelayDto::from(domain_relay)
+            })
+            .collect();
+        Ok(GetAllRelaysResponse::Ok(Json(dtos)))
+    }
+
+    #[oai(path = "/relays/:id/toggle", method = "post")]
+    async fn toggle_relay(&self, id: Path<u8>) -> Result<ToggleRelayResponse> {
+        let relay_id =
+            RelayId::new(*id).map_err(|_| poem::Error::from(ApiError::RelayNotFound(*id)))?;
+        let use_case =
+            ToggleRelayUseCase::new(self.relay_controller.clone(), self.label_repository.clone());
+        let relay = use_case
+            .execute(relay_id)
+            .await
+            .map_err(|e| poem::Error::from(ApiError::from(e)))?;
+        let domain_relay =
+            Relay::with_label(relay.id(), relay.state(), relay.label().unwrap_or_default());
+        Ok(ToggleRelayResponse::Ok(Json(RelayDto::from(domain_relay))))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::sync::Arc;
+
+    use poem::http::StatusCode;
+    use poem_openapi::OpenApiService;
+
+    use crate::{
+        domain::relay::{
+            controller::RelayController,
+            repository::RelayLabelRepository,
+            types::{RelayId, RelayState},
+        },
+        infrastructure::{
+            modbus::mock_controller::MockRelayController,
+            persistence::label_repository::MockRelayLabelRepository,
+        },
+    };
+
+    use super::RelayApi;
+
+    fn make_relay_api(controller: Arc<MockRelayController>) -> poem::test::TestClient<impl poem::Endpoint> {
+        let repo = Arc::new(MockRelayLabelRepository::new());
+        let relay_api = RelayApi::new(controller, repo);
+        let api_service = OpenApiService::new(relay_api, "test", "1.0");
+        let app = poem::Route::new().nest("/api", api_service);
+        poem::test::TestClient::new(app)
+    }
+
+    // -- GET /api/relays --
+
+    #[tokio::test]
+    async fn get_all_relays_returns_200() {
+        let controller = Arc::new(MockRelayController::new());
+        let cli = make_relay_api(controller);
+        let resp = cli.get("/api/relays").send().await;
+        resp.assert_status_is_ok();
+    }
+
+    #[tokio::test]
+    async fn get_all_relays_returns_empty_array_when_no_states() {
+        let controller = Arc::new(MockRelayController::new());
+        let cli = make_relay_api(controller);
+        let resp = cli.get("/api/relays").send().await;
+        resp.assert_status_is_ok();
+        let body: Vec<serde_json::Value> = resp.json().await.value().deserialize();
+        assert!(body.is_empty());
+    }
+
+    #[tokio::test]
+    async fn get_all_relays_returns_all_initialized_relays() {
+        let controller = Arc::new(MockRelayController::new());
+        for i in 1u8..=8 {
+            controller
+                .write_relay_state(RelayId::new(i).unwrap(), RelayState::Off)
+                .await
+                .unwrap();
+        }
+        controller
+            .write_relay_state(RelayId::new(1).unwrap(), RelayState::On)
+            .await
+            .unwrap();
+
+        let cli = make_relay_api(controller);
+        let resp = cli.get("/api/relays").send().await;
+        resp.assert_status_is_ok();
+        let body: Vec<serde_json::Value> = resp.json().await.value().deserialize();
+        assert_eq!(body.len(), 8);
+        assert_eq!(body[0]["id"], 1);
+        assert_eq!(body[0]["state"], "on");
+        assert_eq!(body[1]["id"], 2);
+        assert_eq!(body[1]["state"], "off");
+    }
+
+    // -- POST /api/relays/{id}/toggle --
+
+    #[tokio::test]
+    async fn toggle_relay_with_out_of_range_id_9_returns_404() {
+        let controller = Arc::new(MockRelayController::new());
+        let cli = make_relay_api(controller);
+        let resp = cli.post("/api/relays/9/toggle").send().await;
+        resp.assert_status(StatusCode::NOT_FOUND);
+    }
+
+    #[tokio::test]
+    async fn toggle_relay_with_id_0_returns_404() {
+        let controller = Arc::new(MockRelayController::new());
+        let cli = make_relay_api(controller);
+        let resp = cli.post("/api/relays/0/toggle").send().await;
+        resp.assert_status(StatusCode::NOT_FOUND);
+    }
+
+    #[tokio::test]
+    async fn toggle_relay_toggles_off_to_on_and_returns_200() {
+        let controller = Arc::new(MockRelayController::new());
+        controller
+            .write_relay_state(RelayId::new(1).unwrap(), RelayState::Off)
+            .await
+            .unwrap();
+
+        let cli = make_relay_api(controller);
+        let resp = cli.post("/api/relays/1/toggle").send().await;
+        resp.assert_status_is_ok();
+        let body: serde_json::Value = resp.json().await.value().deserialize();
+        assert_eq!(body["id"], 1);
+        assert_eq!(body["state"], "on");
+    }
+
+    #[tokio::test]
+    async fn toggle_relay_toggles_on_to_off_and_returns_200() {
+        let controller = Arc::new(MockRelayController::new());
+        controller
+            .write_relay_state(RelayId::new(3).unwrap(), RelayState::On)
+            .await
+            .unwrap();
+
+        let cli = make_relay_api(controller);
+        let resp = cli.post("/api/relays/3/toggle").send().await;
+        resp.assert_status_is_ok();
+        let body: serde_json::Value = resp.json().await.value().deserialize();
+        assert_eq!(body["id"], 3);
+        assert_eq!(body["state"], "off");
+    }
+
+    #[tokio::test]
+    async fn toggle_relay_includes_label_in_response() {
+        use crate::domain::relay::types::RelayLabel;
+
+        let controller = Arc::new(MockRelayController::new());
+        controller
+            .write_relay_state(RelayId::new(2).unwrap(), RelayState::Off)
+            .await
+            .unwrap();
+
+        let repo = Arc::new(MockRelayLabelRepository::new());
+        repo.save_label(RelayId::new(2).unwrap(), RelayLabel::new("Pump".to_string()).unwrap())
+            .await
+            .unwrap();
+
+        let relay_api = RelayApi::new(controller, repo);
+        let api_service = OpenApiService::new(relay_api, "test", "1.0");
+        let app = poem::Route::new().nest("/api", api_service);
+        let cli = poem::test::TestClient::new(app);
+
+        let resp = cli.post("/api/relays/2/toggle").send().await;
+        resp.assert_status_is_ok();
+        let body: serde_json::Value = resp.json().await.value().deserialize();
+        assert_eq!(body["label"], "Pump");
+    }
+
+    // -- Integration tests via get_test_app() --
+
+    #[tokio::test]
+    async fn get_all_relays_endpoint_reachable_via_full_app() {
+        let app = crate::get_test_app().await;
+        let cli = poem::test::TestClient::new(app);
+        let resp = cli.get("/api/relays").send().await;
+        resp.assert_status_is_ok();
+    }
+
+    #[tokio::test]
+    async fn toggle_relay_invalid_id_returns_404_via_full_app() {
+        let app = crate::get_test_app().await;
+        let cli = poem::test::TestClient::new(app);
+        let resp = cli.post("/api/relays/9/toggle").send().await;
+        resp.assert_status(StatusCode::NOT_FOUND);
+    }
+
+    // Posting to a valid relay ID on an empty mock should hit the handler (route found)
+    // and return 500 because the mock controller has no relay state initialised.
+    #[tokio::test]
+    async fn toggle_relay_valid_id_empty_mock_returns_500_via_full_app() {
+        let app = crate::get_test_app().await;
+        let cli = poem::test::TestClient::new(app);
+        let resp = cli.post("/api/relays/1/toggle").send().await;
+        resp.assert_status(StatusCode::INTERNAL_SERVER_ERROR);
+    }
+}

backend/src/presentation/dto/mod.rs

@@ -0,0 +1,6 @@
+/// Relay-specific Data Transfer Objects.
+///
+/// This module contains DTO structures for relay-related API responses,
+/// providing serialized representations of relay domain objects for
+/// external consumption.
+pub mod relay_dto;

backend/src/presentation/dto/relay_dto.rs

@@ -0,0 +1,194 @@
+use poem_openapi::Object;
+use serde::{Deserialize, Serialize};
+
+use crate::domain::relay::Relay;
+
+/// Data Transfer Object for relay information.
+///
+/// This struct represents a relay in a serialized format suitable for API
+/// responses. It contains the relay's ID, current state, and label in a
+/// format that can be easily serialized to JSON.
+#[derive(Object, Serialize, Deserialize)]
+pub struct RelayDto {
+    /// The relay's unique identifier (1-8).
+    id: u8,
+    /// The relay's current state as a string ("on" or "off").
+    state: String,
+    /// The relay's user-friendly label.
+    label: String,
+}
+
+impl From<Relay> for RelayDto {
+    /// Converts a domain Relay object to a `RelayDto`.
+    ///
+    /// This conversion extracts the relay's ID, state, and label from the
+    /// domain object and formats them for API consumption.
+    ///
+    /// # Arguments
+    ///
+    /// * `value` - The Relay domain object to convert
+    ///
+    /// # Returns
+    ///
+    /// A `RelayDto` containing the relay's data in serialized format
+    fn from(value: Relay) -> Self {
+        let id = value.id().as_u8();
+        let state = value.state().to_string();
+        let label = value.label().to_string();
+        Self { id, state, label }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::domain::relay::types::{RelayId, RelayLabel, RelayState};
+
+    #[test]
+    fn test_relay_dto_from_relay_with_default_label() {
+        // Test: Relay with default label converts to RelayDto with None label
+        let relay_id = RelayId::new(1).unwrap();
+        let relay = Relay::new(relay_id);
+        let dto = RelayDto::from(relay);
+
+        assert_eq!(dto.id, 1);
+        assert_eq!(dto.state, "off");
+        assert_eq!(dto.label, "Unlabeled".to_string());
+    }
+
+    #[test]
+    fn test_relay_dto_from_relay_with_custom_label() {
+        // Test: Relay with custom label converts to RelayDto with Some(label)
+        let relay_id = RelayId::new(2).unwrap();
+        let label = RelayLabel::new("Water Pump".to_string()).unwrap();
+        let relay = Relay::with_label(relay_id, RelayState::On, label);
+        let dto = RelayDto::from(relay);
+
+        assert_eq!(dto.id, 2);
+        assert_eq!(dto.state, "on");
+        assert_eq!(dto.label, "Water Pump".to_string());
+    }
+
+    #[test]
+    fn test_relay_dto_from_relay_with_on_state() {
+        // Test: Relay with On state converts to RelayDto with "on" state
+        let relay_id = RelayId::new(3).unwrap();
+        let relay = Relay::with_state(relay_id, RelayState::On);
+        let dto = RelayDto::from(relay);
+
+        assert_eq!(dto.id, 3);
+        assert_eq!(dto.state, "on");
+        assert_eq!(dto.label, "Unlabeled".to_string());
+    }
+
+    #[test]
+    fn test_relay_dto_from_relay_with_off_state() {
+        // Test: Relay with Off state converts to RelayDto with "off" state
+        let relay_id = RelayId::new(4).unwrap();
+        let relay = Relay::with_state(relay_id, RelayState::Off);
+        let dto = RelayDto::from(relay);
+
+        assert_eq!(dto.id, 4);
+        assert_eq!(dto.state, "off");
+        assert_eq!(dto.label, "Unlabeled".to_string());
+    }
+
+    #[test]
+    fn test_relay_dto_from_relay_with_max_length_label() {
+        // Test: Relay with maximum length label (50 chars) converts correctly
+        let relay_id = RelayId::new(5).unwrap();
+        let max_label = RelayLabel::new("A".repeat(50)).unwrap();
+        let relay = Relay::with_label(relay_id, RelayState::Off, max_label);
+        let dto = RelayDto::from(relay);
+
+        assert_eq!(dto.id, 5);
+        assert_eq!(dto.state, "off");
+        assert_eq!(dto.label, "A".repeat(50));
+    }
+
+    #[test]
+    fn test_relay_dto_from_relay_with_empty_label_becomes_none() {
+        let relay_id = RelayId::new(6).unwrap();
+        let relay = Relay::new(relay_id);
+        let dto = RelayDto::from(relay);
+
+        assert_eq!(dto.id, 6);
+        assert_eq!(dto.state, "off");
+        assert_eq!(dto.label, "Unlabeled".to_string());
+    }
+
+    #[test]
+    fn test_relay_dto_serialization() {
+        // Test: RelayDto can be serialized to JSON
+        let relay_id = RelayId::new(7).unwrap();
+        let label = RelayLabel::new("Test Relay".to_string()).unwrap();
+        let relay = Relay::with_label(relay_id, RelayState::On, label);
+        let dto = RelayDto::from(relay);
+
+        let json = serde_json::to_string(&dto).unwrap();
+        assert_eq!(json, r#"{"id":7,"state":"on","label":"Test Relay"}"#);
+    }
+
+    #[test]
+    fn test_relay_dto_deserialization() {
+        // Test: RelayDto can be deserialized from JSON
+        let json = r#"{"id":8,"state":"off","label":"Another Relay"}"#;
+        let dto: RelayDto = serde_json::from_str(json).unwrap();
+
+        assert_eq!(dto.id, 8);
+        assert_eq!(dto.state, "off");
+        assert_eq!(dto.label, "Another Relay".to_string());
+    }
+
+    #[test]
+    fn test_relay_dto_all_valid_relay_ids() {
+        // Test: All valid relay IDs (1-8) convert correctly
+        for id_val in 1..=8 {
+            let relay_id = RelayId::new(id_val).unwrap();
+            let relay = Relay::new(relay_id);
+            let dto = RelayDto::from(relay);
+
+            assert_eq!(dto.id, id_val);
+            assert_eq!(dto.state, "off");
+            assert_eq!(dto.label, "Unlabeled".to_string());
+        }
+    }
+
+    #[test]
+    fn test_relay_dto_state_toggle_reflected() {
+        // Test: Relay state changes are reflected in DTO
+        let relay_id = RelayId::new(1).unwrap();
+        let mut relay = Relay::with_state(relay_id, RelayState::Off);
+
+        // Initial state
+        let dto1 = RelayDto::from(relay.clone());
+        assert_eq!(dto1.state, "off");
+
+        // After toggle
+        relay.toggle();
+        let dto2 = RelayDto::from(relay.clone());
+        assert_eq!(dto2.state, "on");
+
+        // After another toggle
+        relay.toggle();
+        let dto3 = RelayDto::from(relay);
+        assert_eq!(dto3.state, "off");
+    }
+
+    #[test]
+    fn test_relay_dto_label_change_reflected() {
+        // Test: Relay label changes are reflected in DTO
+        let relay_id = RelayId::new(2).unwrap();
+        let mut relay = Relay::new(relay_id);
+
+        // Initial label (default)
+        let dto1 = RelayDto::from(relay.clone());
+        assert_eq!(dto1.label, "Unlabeled".to_string());
+
+        // After setting custom label
+        let new_label = RelayLabel::new("Custom Label".to_string()).unwrap();
+        relay.set_label(new_label);
+        let dto2 = RelayDto::from(relay);
+        assert_eq!(dto2.label, "Custom Label".to_string());
+    }
+}

backend/src/presentation/error.rs

@@ -0,0 +1,219 @@
+//! API error types for the presentation layer.
+//!
+//! Defines [`ApiError`], the single error type returned by all API handlers.
+//! Each variant maps to an appropriate HTTP status code via [`poem::error::ResponseError`].
+
+use poem::{error::ResponseError, http::StatusCode};
+
+use crate::{
+    application::use_cases::{get_all_relays::GetAllRelaysError, toggle_relay::ToggleRelayError},
+    domain::relay::{
+        controller::ControllerError, repository::RepositoryError, types::RelayLabelError,
+    },
+};
+
+/// Unified error type for all API handlers.
+///
+/// Variants cover every failure mode that can reach the presentation layer and
+/// map each one to a semantically appropriate HTTP status code.
+#[derive(Debug, thiserror::Error)]
+pub enum ApiError {
+    /// Relay ID is outside the valid range 1-8, error 404
+    #[error("Relay not found: ID {0} is outside the valid range (1-8)")]
+    RelayNotFound(u8),
+    /// Input validation failed (e.g. empty or too long label), error 400
+    #[error("Bad request: {0}")]
+    BadRequest(String),
+    /// Hardware controller failure, error 503 or 504
+    #[error("Controller error: {0}")]
+    ControllerError(#[from] ControllerError),
+    /// Database / repository failure, error 500
+    #[error("Repository error: {0}")]
+    RepositoryError(#[from] RepositoryError),
+}
+
+impl ResponseError for ApiError {
+    fn status(&self) -> poem::http::StatusCode {
+        match self {
+            Self::RelayNotFound(_) => StatusCode::NOT_FOUND,
+            Self::BadRequest(_) => StatusCode::BAD_REQUEST,
+            Self::ControllerError(e) => match e {
+                ControllerError::Timeout(_) => StatusCode::GATEWAY_TIMEOUT,
+                ControllerError::ConnectionError(_) | ControllerError::ModbusException(_) => {
+                    StatusCode::SERVICE_UNAVAILABLE
+                }
+                // InvalidRelayId and InvalidInput are programmer errors at this layer
+                _ => StatusCode::INTERNAL_SERVER_ERROR,
+            },
+            Self::RepositoryError(_) => StatusCode::INTERNAL_SERVER_ERROR,
+        }
+    }
+}
+
+impl From<RelayLabelError> for ApiError {
+    fn from(value: RelayLabelError) -> Self {
+        Self::BadRequest(value.to_string())
+    }
+}
+
+impl From<GetAllRelaysError> for ApiError {
+    fn from(value: GetAllRelaysError) -> Self {
+        match value {
+            GetAllRelaysError::Controller(e) => Self::ControllerError(e),
+            GetAllRelaysError::Repository(e) => Self::RepositoryError(e),
+        }
+    }
+}
+
+impl From<ToggleRelayError> for ApiError {
+    fn from(value: ToggleRelayError) -> Self {
+        match value {
+            ToggleRelayError::Controller(e) => Self::ControllerError(e),
+            ToggleRelayError::Repository(e) => Self::RepositoryError(e),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use poem::error::ResponseError;
+    use poem::http::StatusCode;
+
+    use crate::{
+        application::use_cases::{
+            get_all_relays::GetAllRelaysError, toggle_relay::ToggleRelayError,
+        },
+        domain::relay::{
+            controller::ControllerError,
+            repository::RepositoryError,
+            types::{RelayId, RelayLabelError},
+        },
+    };
+
+    // --- Status code mapping ---
+
+    #[test]
+    fn test_relay_not_found_returns_404() {
+        let error = ApiError::RelayNotFound(9);
+        assert_eq!(error.status(), StatusCode::NOT_FOUND);
+    }
+
+    #[test]
+    fn test_bad_request_returns_400() {
+        let error = ApiError::BadRequest("invalid input".to_string());
+        assert_eq!(error.status(), StatusCode::BAD_REQUEST);
+    }
+
+    #[test]
+    fn test_controller_timeout_returns_504() {
+        let error = ApiError::ControllerError(ControllerError::Timeout(5));
+        assert_eq!(error.status(), StatusCode::GATEWAY_TIMEOUT);
+    }
+
+    #[test]
+    fn test_controller_connection_error_returns_503() {
+        let error =
+            ApiError::ControllerError(ControllerError::ConnectionError("refused".to_string()));
+        assert_eq!(error.status(), StatusCode::SERVICE_UNAVAILABLE);
+    }
+
+    #[test]
+    fn test_controller_modbus_exception_returns_503() {
+        let error = ApiError::ControllerError(ControllerError::ModbusException(
+            "illegal function".to_string(),
+        ));
+        assert_eq!(error.status(), StatusCode::SERVICE_UNAVAILABLE);
+    }
+
+    #[test]
+    fn test_controller_invalid_relay_id_returns_500() {
+        let error = ApiError::ControllerError(ControllerError::InvalidRelayId(9));
+        assert_eq!(error.status(), StatusCode::INTERNAL_SERVER_ERROR);
+    }
+
+    #[test]
+    fn test_controller_invalid_input_returns_500() {
+        let error =
+            ApiError::ControllerError(ControllerError::InvalidInput("bad input".to_string()));
+        assert_eq!(error.status(), StatusCode::INTERNAL_SERVER_ERROR);
+    }
+
+    #[test]
+    fn test_repository_error_returns_500() {
+        let error =
+            ApiError::RepositoryError(RepositoryError::DatabaseError("db failed".to_string()));
+        assert_eq!(error.status(), StatusCode::INTERNAL_SERVER_ERROR);
+    }
+
+    // --- From<RelayLabelError> ---
+
+    #[test]
+    fn test_from_relay_label_error_empty_produces_bad_request() {
+        let api_error = ApiError::from(RelayLabelError::Empty);
+        assert!(matches!(api_error, ApiError::BadRequest(_)));
+    }
+
+    #[test]
+    fn test_from_relay_label_error_too_long_produces_bad_request() {
+        let api_error = ApiError::from(RelayLabelError::TooLong(51));
+        assert!(matches!(api_error, ApiError::BadRequest(_)));
+    }
+
+    // --- From<GetAllRelaysError> ---
+
+    #[test]
+    fn test_from_get_all_relays_controller_error_produces_controller_error() {
+        let source = GetAllRelaysError::Controller(ControllerError::Timeout(5));
+        let api_error = ApiError::from(source);
+        assert!(matches!(api_error, ApiError::ControllerError(_)));
+    }
+
+    #[test]
+    fn test_from_get_all_relays_repository_error_produces_repository_error() {
+        let source =
+            GetAllRelaysError::Repository(RepositoryError::DatabaseError("err".to_string()));
+        let api_error = ApiError::from(source);
+        assert!(matches!(api_error, ApiError::RepositoryError(_)));
+    }
+
+    // --- From<ToggleRelayError> ---
+
+    #[test]
+    fn test_from_toggle_relay_controller_error_produces_controller_error() {
+        let source = ToggleRelayError::Controller(ControllerError::Timeout(5));
+        let api_error = ApiError::from(source);
+        assert!(matches!(api_error, ApiError::ControllerError(_)));
+    }
+
+    #[test]
+    fn test_from_toggle_relay_repository_error_produces_repository_error() {
+        let relay_id = RelayId::new(1).unwrap();
+        let source = ToggleRelayError::Repository(RepositoryError::NotFound(relay_id));
+        let api_error = ApiError::from(source);
+        assert!(matches!(api_error, ApiError::RepositoryError(_)));
+    }
+
+    // --- Error messages ---
+
+    #[test]
+    fn test_relay_not_found_error_message() {
+        let error = ApiError::RelayNotFound(5);
+        assert_eq!(
+            error.to_string(),
+            "Relay not found: ID 5 is outside the valid range (1-8)"
+        );
+    }
+
+    #[test]
+    fn test_bad_request_error_message() {
+        let error = ApiError::BadRequest("invalid label".to_string());
+        assert_eq!(error.to_string(), "Bad request: invalid label");
+    }
+
+    #[test]
+    fn test_relay_label_error_message_preserved_in_bad_request() {
+        let api_error = ApiError::from(RelayLabelError::Empty);
+        assert_eq!(api_error.to_string(), "Bad request: Label cannot be empty");
+    }
+}

backend/src/presentation/mod.rs

@@ -94,3 +94,12 @@
 //! - Architecture: `specs/constitution.md` - API-First Design principle
 //! - API design: `specs/001-modbus-relay-control/plan.md` - Presentation layer tasks
 //! - Domain types: [`crate::domain`] - Types to be wrapped in DTOs
+
+/// Data Transfer Objects (DTOs) for API responses.
+///
+/// This module contains DTO structures that are used to serialize domain
+/// objects for API responses, providing a clean separation between internal
+/// domain models and external API contracts.
+pub mod api;
+pub mod dto;
+pub mod error;

backend/src/route/health.rs

@@ -30,7 +30,7 @@ impl HealthApi {
 
 #[tokio::test]
 async fn health_check_works() {
-    let app = crate::get_test_app();
+    let app = crate::get_test_app().await;
     let cli = poem::test::TestClient::new(app);
     let resp = cli.get("/api/health").send().await;
     resp.assert_status_is_ok();

backend/src/route/meta.rs

@@ -59,7 +59,7 @@ impl MetaApi {
 mod tests {
     #[tokio::test]
     async fn meta_endpoint_returns_correct_data() {
-        let app = crate::get_test_app();
+        let app = crate::get_test_app().await;
         let cli = poem::test::TestClient::new(app);
         let resp = cli.get("/api/meta").send().await;
         resp.assert_status_is_ok();
@@ -78,7 +78,7 @@ mod tests {
 
     #[tokio::test]
     async fn meta_endpoint_returns_200_status() {
-        let app = crate::get_test_app();
+        let app = crate::get_test_app().await;
         let cli = poem::test::TestClient::new(app);
         let resp = cli.get("/api/meta").send().await;
         resp.assert_status_is_ok();

backend/src/route/mod.rs

@@ -12,9 +12,10 @@ mod meta;
 use crate::settings::Settings;
 
 #[derive(Tags)]
-enum ApiCategory {
+pub enum ApiCategory {
     Health,
     Meta,
+    Relays,
 }
 
 pub(crate) struct Api {

backend/src/settings/application.rs

@@ -0,0 +1,16 @@
+/// Application-specific configuration settings.
+#[derive(Debug, serde::Deserialize, Clone, Default)]
+pub struct ApplicationSettings {
+    /// Application name
+    pub name: String,
+    /// Application version
+    pub version: String,
+    /// Port to bind to
+    pub port: u16,
+    /// Host address to bind to
+    pub host: String,
+    /// Base URL of the application
+    pub base_url: String,
+    /// Protocol (http or https)
+    pub protocol: String,
+}

backend/src/settings/database.rs

@@ -0,0 +1,12 @@
+#[derive(Debug, serde::Deserialize, Clone)]
+pub struct DatabaseSettings {
+    pub path: String,
+}
+
+impl Default for DatabaseSettings {
+    fn default() -> Self {
+        Self {
+            path: "sqlite::memory:".to_string(),
+        }
+    }
+}

backend/src/settings/environment.rs

@@ -0,0 +1,134 @@
+/// Application environment.
+#[derive(Debug, PartialEq, Eq, Default)]
+pub enum Environment {
+    /// Development environment
+    #[default]
+    Development,
+    /// Production environment
+    Production,
+}
+
+impl std::fmt::Display for Environment {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        let self_str = match self {
+            Self::Development => "development",
+            Self::Production => "production",
+        };
+        write!(f, "{self_str}")
+    }
+}
+
+impl TryFrom<String> for Environment {
+    type Error = String;
+
+    fn try_from(value: String) -> Result<Self, Self::Error> {
+        Self::try_from(value.as_str())
+    }
+}
+
+impl TryFrom<&str> for Environment {
+    type Error = String;
+
+    fn try_from(value: &str) -> Result<Self, Self::Error> {
+        match value.to_lowercase().as_str() {
+            "development" | "dev" => Ok(Self::Development),
+            "production" | "prod" => Ok(Self::Production),
+            other => Err(format!(
+                "{other} is not a supported environment. Use either `development` or `production`"
+            )),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn environment_display_development() {
+        let env = Environment::Development;
+        assert_eq!(env.to_string(), "development");
+    }
+
+    #[test]
+    fn environment_display_production() {
+        let env = Environment::Production;
+        assert_eq!(env.to_string(), "production");
+    }
+
+    #[test]
+    fn environment_from_str_development() {
+        assert_eq!(
+            Environment::try_from("development").unwrap(),
+            Environment::Development
+        );
+        assert_eq!(
+            Environment::try_from("dev").unwrap(),
+            Environment::Development
+        );
+        assert_eq!(
+            Environment::try_from("Development").unwrap(),
+            Environment::Development
+        );
+        assert_eq!(
+            Environment::try_from("DEV").unwrap(),
+            Environment::Development
+        );
+    }
+
+    #[test]
+    fn environment_from_str_production() {
+        assert_eq!(
+            Environment::try_from("production").unwrap(),
+            Environment::Production
+        );
+        assert_eq!(
+            Environment::try_from("prod").unwrap(),
+            Environment::Production
+        );
+        assert_eq!(
+            Environment::try_from("Production").unwrap(),
+            Environment::Production
+        );
+        assert_eq!(
+            Environment::try_from("PROD").unwrap(),
+            Environment::Production
+        );
+    }
+
+    #[test]
+    fn environment_from_str_invalid() {
+        let result = Environment::try_from("invalid");
+        assert!(result.is_err());
+        assert!(result.unwrap_err().contains("not a supported environment"));
+    }
+
+    #[test]
+    fn environment_from_string_development() {
+        assert_eq!(
+            Environment::try_from("development".to_string()).unwrap(),
+            Environment::Development
+        );
+    }
+
+    #[test]
+    fn environment_from_string_production() {
+        assert_eq!(
+            Environment::try_from("production".to_string()).unwrap(),
+            Environment::Production
+        );
+    }
+
+    #[test]
+    fn environment_from_string_invalid() {
+        let result = Environment::try_from("invalid".to_string());
+        assert!(result.is_err());
+    }
+
+    #[test]
+    fn environment_default_is_development() {
+        let env = Environment::default();
+        assert_eq!(env, Environment::Development);
+    }
+
+}

backend/src/settings/mod.rs

@@ -7,8 +7,21 @@
 //! Settings include application details, Modbus connection parameters, relay configuration,
 //! rate limiting, and environment settings.
 
+mod application;
 mod cors;
+mod database;
+mod environment;
+mod modbus;
+mod rate_limiting;
+mod relay;
+
+pub use application::ApplicationSettings;
 pub use cors::CorsSettings;
+pub use database::DatabaseSettings;
+pub use environment::Environment;
+pub use modbus::ModbusSettings;
+pub use rate_limiting::RateLimitSettings;
+pub use relay::RelaySettings;
 
 /// Application configuration settings.
 ///
@@ -18,15 +31,21 @@ pub struct Settings {
     /// Application-specific settings (name, version, host, port, etc.)
     pub application: ApplicationSettings,
     /// Debug mode flag
+    #[serde(default)]
     pub debug: bool,
     /// Frontend URL for CORS configuration
     pub frontend_url: String,
+    /// Database settings
+    #[serde(default)]
+    pub database: DatabaseSettings,
     /// Rate limiting configuration
     #[serde(default)]
     pub rate_limit: RateLimitSettings,
     /// Modbus configuration
+    #[serde(default)]
     pub modbus: ModbusSettings,
     /// Relay configuration
+    #[serde(default)]
     pub relay: RelaySettings,
     /// CORS configuration
     #[serde(default)]
@@ -78,272 +97,10 @@ impl Settings {
     }
 }
 
-/// Application-specific configuration settings.
-#[derive(Debug, serde::Deserialize, Clone, Default)]
-pub struct ApplicationSettings {
-    /// Application name
-    pub name: String,
-    /// Application version
-    pub version: String,
-    /// Port to bind to
-    pub port: u16,
-    /// Host address to bind to
-    pub host: String,
-    /// Base URL of the application
-    pub base_url: String,
-    /// Protocol (http or https)
-    pub protocol: String,
-}
-
-/// Application environment.
-#[derive(Debug, PartialEq, Eq, Default)]
-pub enum Environment {
-    /// Development environment
-    #[default]
-    Development,
-    /// Production environment
-    Production,
-}
-
-impl std::fmt::Display for Environment {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        let self_str = match self {
-            Self::Development => "development",
-            Self::Production => "production",
-        };
-        write!(f, "{self_str}")
-    }
-}
-
-impl TryFrom<String> for Environment {
-    type Error = String;
-
-    fn try_from(value: String) -> Result<Self, Self::Error> {
-        Self::try_from(value.as_str())
-    }
-}
-
-impl TryFrom<&str> for Environment {
-    type Error = String;
-
-    fn try_from(value: &str) -> Result<Self, Self::Error> {
-        match value.to_lowercase().as_str() {
-            "development" | "dev" => Ok(Self::Development),
-            "production" | "prod" => Ok(Self::Production),
-            other => Err(format!(
-                "{other} is not a supported environment. Use either `development` or `production`"
-            )),
-        }
-    }
-}
-
-/// Rate limiting configuration.
-#[derive(Debug, serde::Deserialize, Clone)]
-pub struct RateLimitSettings {
-    /// Whether rate limiting is enabled
-    #[serde(default = "default_rate_limit_enabled")]
-    pub enabled: bool,
-    /// Maximum number of requests allowed in the time window (burst size)
-    #[serde(default = "default_burst_size")]
-    pub burst_size: u32,
-    /// Time window in seconds for rate limiting
-    #[serde(default = "default_per_seconds")]
-    pub per_seconds: u64,
-}
-
-impl Default for RateLimitSettings {
-    fn default() -> Self {
-        Self {
-            enabled: default_rate_limit_enabled(),
-            burst_size: default_burst_size(),
-            per_seconds: default_per_seconds(),
-        }
-    }
-}
-
-const fn default_rate_limit_enabled() -> bool {
-    true
-}
-
-const fn default_burst_size() -> u32 {
-    100
-}
-
-const fn default_per_seconds() -> u64 {
-    60
-}
-
-/// Modbus TCP connection configuration.
-///
-/// Configures the connection parameters for communicating with the Modbus relay device
-/// using Modbus RTU over TCP protocol.
-#[derive(Debug, serde::Deserialize, Clone)]
-pub struct ModbusSettings {
-    /// IP address or hostname of the Modbus device
-    pub host: String,
-    /// TCP port for Modbus communication (standard Modbus TCP port is 502)
-    pub port: u16,
-    /// Modbus slave/device ID (unit identifier)
-    pub slave_id: u8,
-    /// Operation timeout in seconds
-    pub timeout_secs: u8,
-}
-
-impl Default for ModbusSettings {
-    fn default() -> Self {
-        Self {
-            host: "192.168.0.200".to_string(),
-            port: 502,
-            slave_id: 0,
-            timeout_secs: 5,
-        }
-    }
-}
-
-/// Relay control configuration.
-///
-/// Configures parameters for relay management and labeling.
-#[derive(Debug, serde::Deserialize, Clone)]
-pub struct RelaySettings {
-    /// Maximum length for custom relay labels (in characters)
-    pub label_max_length: u8,
-}
-
-impl Default for RelaySettings {
-    fn default() -> Self {
-        Self {
-            label_max_length: 8,
-        }
-    }
-}
-
 #[cfg(test)]
 mod tests {
     use super::*;
 
-    #[test]
-    fn environment_display_development() {
-        let env = Environment::Development;
-        assert_eq!(env.to_string(), "development");
-    }
-
-    #[test]
-    fn environment_display_production() {
-        let env = Environment::Production;
-        assert_eq!(env.to_string(), "production");
-    }
-
-    #[test]
-    fn environment_from_str_development() {
-        assert_eq!(
-            Environment::try_from("development").unwrap(),
-            Environment::Development
-        );
-        assert_eq!(
-            Environment::try_from("dev").unwrap(),
-            Environment::Development
-        );
-        assert_eq!(
-            Environment::try_from("Development").unwrap(),
-            Environment::Development
-        );
-        assert_eq!(
-            Environment::try_from("DEV").unwrap(),
-            Environment::Development
-        );
-    }
-
-    #[test]
-    fn environment_from_str_production() {
-        assert_eq!(
-            Environment::try_from("production").unwrap(),
-            Environment::Production
-        );
-        assert_eq!(
-            Environment::try_from("prod").unwrap(),
-            Environment::Production
-        );
-        assert_eq!(
-            Environment::try_from("Production").unwrap(),
-            Environment::Production
-        );
-        assert_eq!(
-            Environment::try_from("PROD").unwrap(),
-            Environment::Production
-        );
-    }
-
-    #[test]
-    fn environment_from_str_invalid() {
-        let result = Environment::try_from("invalid");
-        assert!(result.is_err());
-        assert!(result.unwrap_err().contains("not a supported environment"));
-    }
-
-    #[test]
-    fn environment_from_string_development() {
-        assert_eq!(
-            Environment::try_from("development".to_string()).unwrap(),
-            Environment::Development
-        );
-    }
-
-    #[test]
-    fn environment_from_string_production() {
-        assert_eq!(
-            Environment::try_from("production".to_string()).unwrap(),
-            Environment::Production
-        );
-    }
-
-    #[test]
-    fn environment_from_string_invalid() {
-        let result = Environment::try_from("invalid".to_string());
-        assert!(result.is_err());
-    }
-
-    #[test]
-    fn environment_default_is_development() {
-        let env = Environment::default();
-        assert_eq!(env, Environment::Development);
-    }
-
-    #[test]
-    fn rate_limit_settings_default() {
-        let settings = RateLimitSettings::default();
-        assert!(settings.enabled);
-        assert_eq!(settings.burst_size, 100);
-        assert_eq!(settings.per_seconds, 60);
-    }
-
-    #[test]
-    fn rate_limit_settings_deserialize_full() {
-        let json = r#"{"enabled": true, "burst_size": 50, "per_seconds": 30}"#;
-        let settings: RateLimitSettings = serde_json::from_str(json).unwrap();
-        assert!(settings.enabled);
-        assert_eq!(settings.burst_size, 50);
-        assert_eq!(settings.per_seconds, 30);
-    }
-
-    #[test]
-    fn rate_limit_settings_deserialize_partial() {
-        let json = r#"{"enabled": false}"#;
-        let settings: RateLimitSettings = serde_json::from_str(json).unwrap();
-        assert!(!settings.enabled);
-        assert_eq!(settings.burst_size, 100); // default
-        assert_eq!(settings.per_seconds, 60); // default
-    }
-
-    #[test]
-    fn rate_limit_settings_deserialize_empty() {
-        let json = "{}";
-        let settings: RateLimitSettings = serde_json::from_str(json).unwrap();
-        assert!(settings.enabled); // default
-        assert_eq!(settings.burst_size, 100); // default
-        assert_eq!(settings.per_seconds, 60); // default
-    }
-
-    // T009: Integration test for CorsSettings within Settings struct
     #[test]
     fn settings_loads_cors_section_from_yaml() {
         // Create a temporary settings file with CORS configuration
@@ -369,15 +126,6 @@ cors:
     - "http://localhost:5173"
   allow_credentials: false
   max_age_secs: 3600
-
-modbus:
-  host: "192.168.0.200"
-  port: 502
-  slave_id: 0
-  timeout_secs: 5
-
-relay:
-  label_max_length: 50
         "#;
 
         // Use serde_yaml to deserialize directly

backend/src/settings/modbus.rs

@@ -0,0 +1,26 @@
+/// Modbus TCP connection configuration.
+///
+/// Configures the connection parameters for communicating with the Modbus relay device
+/// using Modbus RTU over TCP protocol.
+#[derive(Debug, serde::Deserialize, Clone)]
+pub struct ModbusSettings {
+    /// IP address or hostname of the Modbus device
+    pub host: String,
+    /// TCP port for Modbus communication (standard Modbus TCP port is 502)
+    pub port: u16,
+    /// Modbus slave/device ID (unit identifier)
+    pub slave_id: u8,
+    /// Operation timeout in seconds
+    pub timeout_secs: u8,
+}
+
+impl Default for ModbusSettings {
+    fn default() -> Self {
+        Self {
+            host: "192.168.0.200".to_string(),
+            port: 502,
+            slave_id: 0,
+            timeout_secs: 5,
+        }
+    }
+}

backend/src/settings/rate_limiting.rs

@@ -0,0 +1,75 @@
+/// Rate limiting configuration.
+#[derive(Debug, serde::Deserialize, Clone)]
+pub struct RateLimitSettings {
+    /// Whether rate limiting is enabled
+    #[serde(default = "default_rate_limit_enabled")]
+    pub enabled: bool,
+    /// Maximum number of requests allowed in the time window (burst size)
+    #[serde(default = "default_burst_size")]
+    pub burst_size: u32,
+    /// Time window in seconds for rate limiting
+    #[serde(default = "default_per_seconds")]
+    pub per_seconds: u64,
+}
+
+impl Default for RateLimitSettings {
+    fn default() -> Self {
+        Self {
+            enabled: default_rate_limit_enabled(),
+            burst_size: default_burst_size(),
+            per_seconds: default_per_seconds(),
+        }
+    }
+}
+
+const fn default_rate_limit_enabled() -> bool {
+    true
+}
+
+const fn default_burst_size() -> u32 {
+    100
+}
+
+const fn default_per_seconds() -> u64 {
+    60
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn rate_limit_settings_default() {
+        let settings = RateLimitSettings::default();
+        assert!(settings.enabled);
+        assert_eq!(settings.burst_size, 100);
+        assert_eq!(settings.per_seconds, 60);
+    }
+
+    #[test]
+    fn rate_limit_settings_deserialize_full() {
+        let json = r#"{"enabled": true, "burst_size": 50, "per_seconds": 30}"#;
+        let settings: RateLimitSettings = serde_json::from_str(json).unwrap();
+        assert!(settings.enabled);
+        assert_eq!(settings.burst_size, 50);
+        assert_eq!(settings.per_seconds, 30);
+    }
+
+    #[test]
+    fn rate_limit_settings_deserialize_partial() {
+        let json = r#"{"enabled": false}"#;
+        let settings: RateLimitSettings = serde_json::from_str(json).unwrap();
+        assert!(!settings.enabled);
+        assert_eq!(settings.burst_size, 100); // default
+        assert_eq!(settings.per_seconds, 60); // default
+    }
+
+    #[test]
+    fn rate_limit_settings_deserialize_empty() {
+        let json = "{}";
+        let settings: RateLimitSettings = serde_json::from_str(json).unwrap();
+        assert!(settings.enabled); // default
+        assert_eq!(settings.burst_size, 100); // default
+        assert_eq!(settings.per_seconds, 60); // default
+    }
+}

backend/src/settings/relay.rs

@@ -0,0 +1,16 @@
+/// Relay control configuration.
+///
+/// Configures parameters for relay management and labeling.
+#[derive(Debug, serde::Deserialize, Clone)]
+pub struct RelaySettings {
+    /// Maximum length for custom relay labels (in characters)
+    pub label_max_length: u8,
+}
+
+impl Default for RelaySettings {
+    fn default() -> Self {
+        Self {
+            label_max_length: 8,
+        }
+    }
+}

backend/src/startup.rs

@@ -10,6 +10,9 @@ use poem::middleware::{AddDataEndpoint, Cors, CorsEndpoint};
 use poem::{EndpointExt, Route};
 use poem_openapi::OpenApiService;
 
+use crate::infrastructure::modbus::factory::create_relay_controller;
+use crate::infrastructure::persistence::factory::create_label_repository;
+use crate::presentation::api::relay_api::RelayApi;
 use crate::{
     middleware::rate_limit::{RateLimit, RateLimitConfig},
     route::Api,
@@ -94,17 +97,17 @@ impl From<Application> for RunnableApplication {
 }
 
 impl Application {
-    fn setup_app(settings: &Settings) -> poem::Route {
+    fn setup_app(settings: &Settings, relay_api: RelayApi) -> poem::Route {
         let api_service = OpenApiService::new(
-            Api::from(settings).apis(),
+            (Api::from(settings).apis(), relay_api),
             settings.application.clone().name,
             settings.application.clone().version,
         )
         .url_prefix("/api");
         let ui = api_service.swagger_ui();
         poem::Route::new()
-            .nest("/api", api_service.clone())
             .nest("/specs", api_service.spec_endpoint_yaml())
+            .nest("/api", api_service)
             .nest("/", ui)
     }
 
@@ -125,22 +128,31 @@ impl Application {
     /// Builds a new application with the given settings and optional TCP listener.
     ///
     /// If no listener is provided, one will be created based on the settings.
-    #[must_use]
-    pub fn build(
+    ///
+    /// # Errors
+    ///
+    /// Returns an error if dependency injection fails (currently always succeeds).
+    pub async fn build(
         settings: Settings,
         tcp_listener: Option<poem::listener::TcpListener<String>>,
-    ) -> Self {
+    ) -> Result<Self, Box<dyn std::error::Error>> {
+        let use_mock = cfg!(test) || std::env::var("CI").is_ok();
+        let relay_controller = create_relay_controller(&settings.modbus, use_mock).await;
+        let label_repository = create_label_repository(&settings.database.path, use_mock).await?;
+        let relay_api = RelayApi::new(relay_controller, label_repository);
+
         let port = settings.application.port;
         let host = settings.application.clone().host;
-        let app = Self::setup_app(&settings);
+        let app = Self::setup_app(&settings, relay_api);
         let server = Self::setup_server(&settings, tcp_listener);
-        Self {
+
+        Ok(Self {
             server,
             app,
             host,
             port,
             settings,
-        }
+        })
     }
 
     /// Converts the application into a runnable application.
@@ -187,67 +199,131 @@ mod tests {
         }
     }
 
-    #[test]
-    fn application_build_and_host() {
+    #[tokio::test]
+    async fn application_build_and_host() {
         let settings = create_test_settings();
-        let app = Application::build(settings.clone(), None);
+        let app = Application::build(settings.clone(), None).await.unwrap();
         assert_eq!(app.host(), settings.application.host);
     }
 
-    #[test]
-    fn application_build_and_port() {
+    #[tokio::test]
+    async fn application_build_and_port() {
         let settings = create_test_settings();
-        let app = Application::build(settings, None);
+        let app = Application::build(settings, None).await.unwrap();
         assert_eq!(app.port(), 8080);
     }
 
-    #[test]
-    fn application_host_returns_correct_value() {
+    #[tokio::test]
+    async fn application_host_returns_correct_value() {
         let settings = create_test_settings();
-        let app = Application::build(settings, None);
+        let app = Application::build(settings, None).await.unwrap();
         assert_eq!(app.host(), "127.0.0.1");
     }
 
-    #[test]
-    fn application_port_returns_correct_value() {
+    #[tokio::test]
+    async fn application_port_returns_correct_value() {
         let settings = create_test_settings();
-        let app = Application::build(settings, None);
+        let app = Application::build(settings, None).await.unwrap();
         assert_eq!(app.port(), 8080);
     }
 
-    #[test]
-    fn application_with_custom_listener() {
+    #[tokio::test]
+    async fn application_with_custom_listener() {
         let settings = create_test_settings();
         let tcp_listener =
             std::net::TcpListener::bind("127.0.0.1:0").expect("Failed to bind random port");
         let port = tcp_listener.local_addr().unwrap().port();
         let listener = poem::listener::TcpListener::bind(format!("127.0.0.1:{port}"));
 
-        let app = Application::build(settings, Some(listener));
+        let app = Application::build(settings, Some(listener)).await.unwrap();
         assert_eq!(app.host(), "127.0.0.1");
         assert_eq!(app.port(), 8080);
     }
 
-    // T015: Test that CORS middleware is configured from settings
-    #[test]
-    fn runnable_application_uses_cors_from_settings() {
-        // GIVEN: An application with custom CORS settings
+    #[tokio::test]
+    async fn runnable_application_uses_cors_from_settings() {
         let mut settings = create_test_settings();
         settings.cors = crate::settings::CorsSettings {
             allowed_origins: vec!["http://localhost:5173".to_string()],
             allow_credentials: false,
             max_age_secs: 3600,
         };
-
-        // WHEN: The application is converted to a runnable application
-        let app = Application::build(settings, None);
+        let app = Application::build(settings, None).await.unwrap();
         let _runnable_app = app.make_app();
-
-        // THEN: The middleware chain should use CORS settings from configuration
         // Note: This is a structural test - actual CORS behavior is tested in integration tests (T016)
         // The fact that this compiles and runs without panic verifies that:
         // 1. CORS settings are properly loaded
         // 2. The From<CorsSettings> trait is correctly implemented
         // 3. The middleware chain accepts the CORS configuration
     }
+
+    #[tokio::test]
+    async fn test_application_build_succeeds_in_test_mode() {
+        let settings = create_test_settings();
+        let app = Application::build(settings, None).await;
+        assert!(
+            app.is_ok(),
+            "Application::build() should succeed in test mode"
+        );
+        let app = app.unwrap();
+        assert_eq!(app.port(), 8080);
+        assert_eq!(app.host(), "127.0.0.1");
+        let runnable_app = app.make_app();
+        let _app: App = runnable_app.into();
+        // Success - the application was built with dependencies and can run
+    }
+
+    // ============================================================================
+    // T039d: RelayApi Registration Tests
+    // ============================================================================
+    // These tests verify that the RelayApi is properly registered in the route
+    // aggregator with correct OpenAPI tagging.
+
+    // T039d: Test 1 - OpenAPI spec includes /relays endpoints
+    #[tokio::test]
+    async fn test_openapi_spec_includes_relay_endpoints() {
+        let settings = create_test_settings();
+        let app: App = Application::build(settings, None)
+            .await
+            .unwrap()
+            .make_app()
+            .into();
+        let cli = poem::test::TestClient::new(app);
+
+        let resp = cli.get("/specs").send().await;
+        resp.assert_status_is_ok();
+
+        let spec = resp.0.into_body().into_string().await.unwrap();
+
+        assert!(
+            spec.contains("/relays:"),
+            "OpenAPI spec should include the /relays path, got:\n{spec}"
+        );
+        assert!(
+            spec.contains("/relays/{id}/toggle:"),
+            "OpenAPI spec should include the /relays/{{id}}/toggle path, got:\n{spec}"
+        );
+    }
+
+    // T039d: Test 2 - OpenAPI spec includes the Relays tag
+    #[tokio::test]
+    async fn test_swagger_ui_includes_relays_tag() {
+        let settings = create_test_settings();
+        let app: App = Application::build(settings, None)
+            .await
+            .unwrap()
+            .make_app()
+            .into();
+        let cli = poem::test::TestClient::new(app);
+
+        let resp = cli.get("/specs").send().await;
+        resp.assert_status_is_ok();
+
+        let spec = resp.0.into_body().into_string().await.unwrap();
+
+        assert!(
+            spec.contains("Relays"),
+            "OpenAPI spec should include a 'Relays' tag, got:\n{spec}"
+        );
+    }
 }

backend/tests/contract/test_relay_api.rs

@@ -0,0 +1,271 @@
+//! Contract tests for the Relay API HTTP endpoints.
+//!
+//! - **T048**: `GET /api/relays` contract tests
+//! - **T050**: `POST /api/relays/:id/toggle` contract tests
+
+use std::sync::Arc;
+
+use poem::{http::StatusCode, test::TestClient};
+use poem_openapi::OpenApiService;
+use sta::{
+    domain::relay::{
+        controller::RelayController,
+        repository::RelayLabelRepository,
+        types::{RelayId, RelayLabel, RelayState},
+    },
+    infrastructure::{
+        modbus::mock_controller::MockRelayController,
+        persistence::label_repository::MockRelayLabelRepository,
+    },
+    presentation::api::relay_api::RelayApi,
+};
+
+// -- Helpers --
+
+fn build_test_client(
+    controller: Arc<MockRelayController>,
+    repo: Arc<MockRelayLabelRepository>,
+) -> TestClient<impl poem::Endpoint> {
+    let relay_api = RelayApi::new(controller, repo);
+    let api_service = OpenApiService::new(relay_api, "STA", "0.1");
+    let app = poem::Route::new().nest("/api", api_service);
+    TestClient::new(app)
+}
+
+/// Creates a controller with all 8 relays initialised to `Off`.
+async fn all_relays_off() -> Arc<MockRelayController> {
+    let controller = Arc::new(MockRelayController::new());
+    for id in 1u8..=8 {
+        controller
+            .write_relay_state(RelayId::new(id).unwrap(), RelayState::Off)
+            .await
+            .unwrap();
+    }
+    controller
+}
+
+// ===========================================================================
+// T048: GET /api/relays
+// ===========================================================================
+
+/// T048 – Returns 200 OK.
+#[tokio::test]
+async fn get_all_relays_returns_200() {
+    let cli = build_test_client(all_relays_off().await, Arc::new(MockRelayLabelRepository::new()));
+
+    let resp = cli.get("/api/relays").send().await;
+
+    resp.assert_status_is_ok();
+}
+
+/// T048 – Returns an array of exactly 8 `RelayDto` objects.
+#[tokio::test]
+async fn get_all_relays_returns_array_of_8_relay_dtos() {
+    let cli = build_test_client(all_relays_off().await, Arc::new(MockRelayLabelRepository::new()));
+
+    let resp = cli.get("/api/relays").send().await;
+    resp.assert_status_is_ok();
+
+    let body: Vec<serde_json::Value> = resp.json().await.value().deserialize();
+    assert_eq!(body.len(), 8, "Expected 8 relays, got {}", body.len());
+}
+
+/// T048 – Relay IDs are 1 through 8, in ascending order.
+#[tokio::test]
+async fn get_all_relays_relay_ids_are_1_to_8_in_order() {
+    let cli = build_test_client(all_relays_off().await, Arc::new(MockRelayLabelRepository::new()));
+
+    let resp = cli.get("/api/relays").send().await;
+    let body: Vec<serde_json::Value> = resp.json().await.value().deserialize();
+
+    for (index, relay) in body.iter().enumerate() {
+        let expected_id = index + 1;
+        assert_eq!(
+            relay["id"], expected_id,
+            "Relay at index {index} should have id {expected_id}"
+        );
+    }
+}
+
+/// T048 – Every relay has a `state` field that is either `"on"` or `"off"`.
+#[tokio::test]
+async fn get_all_relays_each_relay_has_valid_state_field() {
+    let cli = build_test_client(all_relays_off().await, Arc::new(MockRelayLabelRepository::new()));
+
+    let resp = cli.get("/api/relays").send().await;
+    let body: Vec<serde_json::Value> = resp.json().await.value().deserialize();
+
+    for relay in &body {
+        let state = relay["state"].as_str().expect("state should be a string");
+        assert!(
+            state == "on" || state == "off",
+            "state must be 'on' or 'off', got '{state}'"
+        );
+    }
+}
+
+/// T048 – Every relay has a `label` field (string).
+#[tokio::test]
+async fn get_all_relays_each_relay_has_label_field() {
+    let cli = build_test_client(all_relays_off().await, Arc::new(MockRelayLabelRepository::new()));
+
+    let resp = cli.get("/api/relays").send().await;
+    let body: Vec<serde_json::Value> = resp.json().await.value().deserialize();
+
+    for relay in &body {
+        assert!(relay["label"].is_string(), "label should be a string field");
+    }
+}
+
+/// T048 – Relay states in the response match the controller's actual states.
+#[tokio::test]
+async fn get_all_relays_states_reflect_controller_state() {
+    let controller = all_relays_off().await;
+    controller
+        .write_relay_state(RelayId::new(1).unwrap(), RelayState::On)
+        .await
+        .unwrap();
+    controller
+        .write_relay_state(RelayId::new(3).unwrap(), RelayState::On)
+        .await
+        .unwrap();
+
+    let cli = build_test_client(controller, Arc::new(MockRelayLabelRepository::new()));
+    let resp = cli.get("/api/relays").send().await;
+    let body: Vec<serde_json::Value> = resp.json().await.value().deserialize();
+
+    assert_eq!(body[0]["state"], "on",  "Relay 1 should be on");
+    assert_eq!(body[1]["state"], "off", "Relay 2 should be off");
+    assert_eq!(body[2]["state"], "on",  "Relay 3 should be on");
+    assert_eq!(body[3]["state"], "off", "Relay 4 should be off");
+}
+
+/// T048 – A relay with a persisted label returns that label.
+#[tokio::test]
+async fn get_all_relays_relay_with_label_returns_label() {
+    let repo = Arc::new(MockRelayLabelRepository::new());
+    repo.save_label(
+        RelayId::new(2).unwrap(),
+        RelayLabel::new("Water Pump".to_string()).unwrap(),
+    )
+    .await
+    .unwrap();
+
+    let cli = build_test_client(all_relays_off().await, repo);
+    let resp = cli.get("/api/relays").send().await;
+    let body: Vec<serde_json::Value> = resp.json().await.value().deserialize();
+
+    assert_eq!(body[1]["label"], "Water Pump");
+}
+
+// ===========================================================================
+// T050: POST /api/relays/:id/toggle
+// ===========================================================================
+
+/// T050 – Returns 200 OK with a `RelayDto` body.
+#[tokio::test]
+async fn toggle_relay_returns_200_with_relay_dto() {
+    let cli = build_test_client(all_relays_off().await, Arc::new(MockRelayLabelRepository::new()));
+
+    let resp = cli.post("/api/relays/1/toggle").send().await;
+
+    resp.assert_status_is_ok();
+    let body: serde_json::Value = resp.json().await.value().deserialize();
+    assert!(body["id"].is_number());
+    assert!(body["state"].is_string());
+    assert!(body["label"].is_string());
+}
+
+/// T050 – Returns 404 for relay id 0 (below valid range).
+#[tokio::test]
+async fn toggle_relay_returns_404_for_id_below_range() {
+    let cli = build_test_client(all_relays_off().await, Arc::new(MockRelayLabelRepository::new()));
+
+    let resp = cli.post("/api/relays/0/toggle").send().await;
+
+    resp.assert_status(StatusCode::NOT_FOUND);
+}
+
+/// T050 – Returns 404 for relay id 9 (above valid range).
+#[tokio::test]
+async fn toggle_relay_returns_404_for_id_above_range() {
+    let cli = build_test_client(all_relays_off().await, Arc::new(MockRelayLabelRepository::new()));
+
+    let resp = cli.post("/api/relays/9/toggle").send().await;
+
+    resp.assert_status(StatusCode::NOT_FOUND);
+}
+
+/// T050 – State changes from `Off` to `On` and response reflects new state.
+#[tokio::test]
+async fn toggle_relay_off_to_on_response_shows_on() {
+    let cli = build_test_client(all_relays_off().await, Arc::new(MockRelayLabelRepository::new()));
+
+    let resp = cli.post("/api/relays/1/toggle").send().await;
+    resp.assert_status_is_ok();
+
+    let body: serde_json::Value = resp.json().await.value().deserialize();
+    assert_eq!(body["state"], "on");
+}
+
+/// T050 – State changes from `On` to `Off` and response reflects new state.
+#[tokio::test]
+async fn toggle_relay_on_to_off_response_shows_off() {
+    let controller = Arc::new(MockRelayController::new());
+    controller
+        .write_relay_state(RelayId::new(5).unwrap(), RelayState::On)
+        .await
+        .unwrap();
+
+    let cli = build_test_client(controller, Arc::new(MockRelayLabelRepository::new()));
+
+    let resp = cli.post("/api/relays/5/toggle").send().await;
+    resp.assert_status_is_ok();
+
+    let body: serde_json::Value = resp.json().await.value().deserialize();
+    assert_eq!(body["state"], "off");
+}
+
+/// T050 – State actually changes in the underlying controller, not just in the response.
+#[tokio::test]
+async fn toggle_relay_state_actually_changes_in_controller() {
+    let controller = all_relays_off().await;
+    let relay_id = RelayId::new(3).unwrap();
+
+    let cli = build_test_client(controller.clone(), Arc::new(MockRelayLabelRepository::new()));
+    cli.post("/api/relays/3/toggle").send().await;
+
+    let state = controller.read_relay_state(relay_id).await.unwrap();
+    assert_eq!(state, RelayState::On, "Relay 3 should be On in the controller after toggle");
+}
+
+/// T050 – Response includes the correct relay id.
+#[tokio::test]
+async fn toggle_relay_response_includes_correct_relay_id() {
+    let cli = build_test_client(all_relays_off().await, Arc::new(MockRelayLabelRepository::new()));
+
+    let resp = cli.post("/api/relays/4/toggle").send().await;
+    resp.assert_status_is_ok();
+
+    let body: serde_json::Value = resp.json().await.value().deserialize();
+    assert_eq!(body["id"], 4);
+}
+
+/// T050 – Response includes a persisted label.
+#[tokio::test]
+async fn toggle_relay_response_includes_label_when_set() {
+    let repo = Arc::new(MockRelayLabelRepository::new());
+    repo.save_label(
+        RelayId::new(6).unwrap(),
+        RelayLabel::new("Heater".to_string()).unwrap(),
+    )
+    .await
+    .unwrap();
+
+    let cli = build_test_client(all_relays_off().await, repo);
+    let resp = cli.post("/api/relays/6/toggle").send().await;
+    resp.assert_status_is_ok();
+
+    let body: serde_json::Value = resp.json().await.value().deserialize();
+    assert_eq!(body["label"], "Heater");
+}

backend/tests/cors_test.rs

@@ -13,7 +13,7 @@ use poem::test::TestClient;
 use sta::{settings::Settings, startup::Application};
 
 /// Helper function to create a test app with custom CORS settings.
-fn get_test_app_with_cors(
+async fn get_test_app_with_cors(
     allowed_origins: Vec<String>,
     allow_credentials: bool,
     max_age_secs: i32,
@@ -32,6 +32,8 @@ fn get_test_app_with_cors(
     settings.cors.max_age_secs = max_age_secs;
 
     Application::build(settings, Some(listener))
+        .await
+        .expect("Failed to build application")
         .make_app()
         .into()
 }
@@ -42,7 +44,7 @@ fn get_test_app_with_cors(
 #[tokio::test]
 async fn preflight_request_returns_cors_headers() {
     // GIVEN: An app with CORS configured for specific origin
-    let app = get_test_app_with_cors(vec!["http://localhost:5173".to_string()], false, 3600);
+    let app = get_test_app_with_cors(vec!["http://localhost:5173".to_string()], false, 3600).await;
     let client = TestClient::new(app);
 
     // WHEN: A preflight OPTIONS request is sent with Origin header
@@ -82,7 +84,7 @@ async fn preflight_request_returns_cors_headers() {
 #[tokio::test]
 async fn get_request_with_origin_returns_allow_origin_header() {
     // GIVEN: An app with CORS configured for specific origin
-    let app = get_test_app_with_cors(vec!["http://localhost:5173".to_string()], false, 3600);
+    let app = get_test_app_with_cors(vec!["http://localhost:5173".to_string()], false, 3600).await;
     let client = TestClient::new(app);
 
     // WHEN: A GET request is sent with Origin header
@@ -119,7 +121,7 @@ async fn preflight_response_includes_max_age_from_config() {
         vec!["http://localhost:5173".to_string()],
         false,
         custom_max_age,
-    );
+    ).await;
     let client = TestClient::new(app);
 
     // WHEN: A preflight OPTIONS request is sent
@@ -153,7 +155,7 @@ async fn response_includes_allow_credentials_when_configured() {
         vec!["http://localhost:5173".to_string()],
         true, // allow_credentials
         3600,
-    );
+    ).await;
     let client = TestClient::new(app);
 
     // WHEN: A preflight OPTIONS request is sent
@@ -187,7 +189,7 @@ async fn response_does_not_include_credentials_when_disabled() {
         vec!["http://localhost:5173".to_string()],
         false, // allow_credentials
         3600,
-    );
+    ).await;
     let client = TestClient::new(app);
 
     // WHEN: A preflight OPTIONS request is sent
@@ -217,7 +219,7 @@ async fn response_does_not_include_credentials_when_disabled() {
 #[tokio::test]
 async fn preflight_response_includes_correct_allowed_methods() {
     // GIVEN: An app with CORS configured
-    let app = get_test_app_with_cors(vec!["http://localhost:5173".to_string()], false, 3600);
+    let app = get_test_app_with_cors(vec!["http://localhost:5173".to_string()], false, 3600).await;
     let client = TestClient::new(app);
 
     // WHEN: A preflight OPTIONS request is sent
@@ -260,7 +262,7 @@ async fn wildcard_origin_works_with_credentials_disabled() {
         vec!["*".to_string()],
         false, // credentials MUST be false with wildcard
         3600,
-    );
+    ).await;
     let client = TestClient::new(app);
 
     // WHEN: A preflight OPTIONS request is sent with any origin
@@ -299,7 +301,7 @@ async fn multiple_origins_are_supported() {
         ],
         false,
         3600,
-    );
+    ).await;
     let client = TestClient::new(app);
 
     // WHEN: A request is sent with the first origin
@@ -341,7 +343,7 @@ async fn multiple_origins_are_supported() {
 #[tokio::test]
 async fn unauthorized_origin_is_rejected() {
     // GIVEN: An app with CORS configured for specific origins only
-    let app = get_test_app_with_cors(vec!["http://localhost:5173".to_string()], false, 3600);
+    let app = get_test_app_with_cors(vec!["http://localhost:5173".to_string()], false, 3600).await;
     let client = TestClient::new(app);
 
     // WHEN: A request is sent with an unauthorized origin

backend/tests/sqlite_repository_functional_test.rs

@@ -427,7 +427,10 @@ async fn test_repository_error_handling() {
 
     // Test with invalid relay ID (should be caught by domain validation)
     let invalid_relay_id = RelayId::new(9); // This will fail validation
-    assert!(invalid_relay_id.is_err(), "Invalid relay ID should fail validation");
+    assert!(
+        invalid_relay_id.is_err(),
+        "Invalid relay ID should fail validation"
+    );
 
     // Test with invalid label (should be caught by domain validation)
     let invalid_label = RelayLabel::new("".to_string()); // Empty label

flake.lock

@@ -23,76 +23,6 @@
         "type": "github"
       }
     },
-    "cachix": {
-      "inputs": {
-        "devenv": [
-          "devenv"
-        ],
-        "flake-compat": [
-          "devenv",
-          "flake-compat"
-        ],
-        "git-hooks": [
-          "devenv",
-          "git-hooks"
-        ],
-        "nixpkgs": [
-          "devenv",
-          "nixpkgs"
-        ]
-      },
-      "locked": {
-        "lastModified": 1760971495,
-        "narHash": "sha256-IwnNtbNVrlZIHh7h4Wz6VP0Furxg9Hh0ycighvL5cZc=",
-        "owner": "cachix",
-        "repo": "cachix",
-        "rev": "c5bfd933d1033672f51a863c47303fc0e093c2d2",
-        "type": "github"
-      },
-      "original": {
-        "owner": "cachix",
-        "ref": "latest",
-        "repo": "cachix",
-        "type": "github"
-      }
-    },
-    "devenv": {
-      "inputs": {
-        "cachix": "cachix",
-        "flake-compat": "flake-compat",
-        "flake-parts": "flake-parts",
-        "git-hooks": "git-hooks",
-        "nix": "nix",
-        "nixpkgs": [
-          "nixpkgs"
-        ]
-      },
-      "locked": {
-        "lastModified": 1766843567,
-        "narHash": "sha256-062oL6KZCH7ePf4BBG61OdFJUh5ovw6zTpd/lVwy/xk=",
-        "owner": "cachix",
-        "repo": "devenv",
-        "rev": "d0f2c8545f09e5aba9d321079a284b550371879d",
-        "type": "github"
-      },
-      "original": {
-        "owner": "cachix",
-        "repo": "devenv",
-        "type": "github"
-      }
-    },
-    "devenv-root": {
-      "flake": false,
-      "locked": {
-        "narHash": "sha256-d6xi4mKdjkX2JFicDIv5niSzpyI0m/Hnm8GGAIU04kY=",
-        "type": "file",
-        "url": "file:///dev/null"
-      },
-      "original": {
-        "type": "file",
-        "url": "file:///dev/null"
-      }
-    },
     "fenix": {
       "inputs": {
         "nixpkgs": [
@@ -115,43 +45,6 @@
         "type": "github"
       }
     },
-    "flake-compat": {
-      "flake": false,
-      "locked": {
-        "lastModified": 1761588595,
-        "narHash": "sha256-XKUZz9zewJNUj46b4AJdiRZJAvSZ0Dqj2BNfXvFlJC4=",
-        "owner": "edolstra",
-        "repo": "flake-compat",
-        "rev": "f387cd2afec9419c8ee37694406ca490c3f34ee5",
-        "type": "github"
-      },
-      "original": {
-        "owner": "edolstra",
-        "repo": "flake-compat",
-        "type": "github"
-      }
-    },
-    "flake-parts": {
-      "inputs": {
-        "nixpkgs-lib": [
-          "devenv",
-          "nixpkgs"
-        ]
-      },
-      "locked": {
-        "lastModified": 1760948891,
-        "narHash": "sha256-TmWcdiUUaWk8J4lpjzu4gCGxWY6/Ok7mOK4fIFfBuU4=",
-        "owner": "hercules-ci",
-        "repo": "flake-parts",
-        "rev": "864599284fc7c0ba6357ed89ed5e2cd5040f0c04",
-        "type": "github"
-      },
-      "original": {
-        "owner": "hercules-ci",
-        "repo": "flake-parts",
-        "type": "github"
-      }
-    },
     "flake-utils": {
       "inputs": {
         "systems": "systems"
@@ -186,115 +79,25 @@
         "type": "github"
       }
     },
-    "git-hooks": {
-      "inputs": {
-        "flake-compat": [
-          "devenv",
-          "flake-compat"
-        ],
-        "gitignore": "gitignore",
-        "nixpkgs": [
-          "devenv",
-          "nixpkgs"
-        ]
-      },
-      "locked": {
-        "lastModified": 1760663237,
-        "narHash": "sha256-BflA6U4AM1bzuRMR8QqzPXqh8sWVCNDzOdsxXEguJIc=",
-        "owner": "cachix",
-        "repo": "git-hooks.nix",
-        "rev": "ca5b894d3e3e151ffc1db040b6ce4dcc75d31c37",
-        "type": "github"
-      },
-      "original": {
-        "owner": "cachix",
-        "repo": "git-hooks.nix",
-        "type": "github"
-      }
-    },
-    "gitignore": {
-      "inputs": {
-        "nixpkgs": [
-          "devenv",
-          "git-hooks",
-          "nixpkgs"
-        ]
-      },
-      "locked": {
-        "lastModified": 1709087332,
-        "narHash": "sha256-HG2cCnktfHsKV0s4XW83gU3F57gaTljL9KNSuG6bnQs=",
-        "owner": "hercules-ci",
-        "repo": "gitignore.nix",
-        "rev": "637db329424fd7e46cf4185293b9cc8c88c95394",
-        "type": "github"
-      },
-      "original": {
-        "owner": "hercules-ci",
-        "repo": "gitignore.nix",
-        "type": "github"
-      }
-    },
-    "nix": {
-      "inputs": {
-        "flake-compat": [
-          "devenv",
-          "flake-compat"
-        ],
-        "flake-parts": [
-          "devenv",
-          "flake-parts"
-        ],
-        "git-hooks-nix": [
-          "devenv",
-          "git-hooks"
-        ],
-        "nixpkgs": [
-          "devenv",
-          "nixpkgs"
-        ],
-        "nixpkgs-23-11": [
-          "devenv"
-        ],
-        "nixpkgs-regression": [
-          "devenv"
-        ]
-      },
-      "locked": {
-        "lastModified": 1761648602,
-        "narHash": "sha256-H97KSB/luq/aGobKRuHahOvT1r7C03BgB6D5HBZsbN8=",
-        "owner": "cachix",
-        "repo": "nix",
-        "rev": "3e5644da6830ef65f0a2f7ec22830c46285bfff6",
-        "type": "github"
-      },
-      "original": {
-        "owner": "cachix",
-        "ref": "devenv-2.30.6",
-        "repo": "nix",
-        "type": "github"
-      }
-    },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1764580874,
-        "narHash": "sha256-GMlWyeVh6fVuPeJI+ZmbJVV8DDS5wfdfDY88FHt5g/8=",
-        "owner": "cachix",
-        "repo": "devenv-nixpkgs",
-        "rev": "dcf61356c3ab25f1362b4a4428a6d871e84f1d1d",
+        "lastModified": 1777954456,
+        "narHash": "sha256-hGdgeU2Nk87RAuZyYjyDjFL6LK7dAZN5RE9+hrDTkDU=",
+        "owner": "nixos",
+        "repo": "nixpkgs",
+        "rev": "549bd84d6279f9852cae6225e372cc67fb91a4c1",
         "type": "github"
       },
       "original": {
-        "owner": "cachix",
-        "ref": "rolling",
-        "repo": "devenv-nixpkgs",
+        "owner": "nixos",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
         "type": "github"
       }
     },
     "root": {
       "inputs": {
         "alejandra": "alejandra",
-        "devenv": "devenv",
-        "devenv-root": "devenv-root",
         "flake-utils": "flake-utils",
         "nixpkgs": "nixpkgs",
         "rust-overlay": "rust-overlay"
@@ -324,11 +127,11 @@
         ]
       },
       "locked": {
-        "lastModified": 1766803264,
-        "narHash": "sha256-eGK6He8BR6L7N73kyyjz/vGxZX1Usnr8Gwfs3D18KgE=",
+        "lastModified": 1777950921,
+        "narHash": "sha256-NpOgt8ISaHTDNJZjNUfwFfbieKfRXzab4WKM31gZCGA=",
         "owner": "oxalica",
         "repo": "rust-overlay",
-        "rev": "6b5c52313aaf3f3e1a0a6757bb89846edfb5195c",
+        "rev": "366ea19e0e55b768f74b7a0b2a20f847e7ae828d",
         "type": "github"
       },
       "original": {

flake.nix

@@ -1,57 +1,69 @@
 {
   inputs = {
-    nixpkgs.url = "github:cachix/devenv-nixpkgs/rolling";
+    nixpkgs.url = "github:nixos/nixpkgs?ref=nixos-unstable";
     flake-utils.url = "github:numtide/flake-utils";
     alejandra = {
       url = "github:kamadorueda/alejandra/4.0.0";
       inputs.nixpkgs.follows = "nixpkgs";
     };
-    devenv = {
-      url = "github:cachix/devenv";
-      inputs.nixpkgs.follows = "nixpkgs";
-    };
     rust-overlay = {
       url = "github:oxalica/rust-overlay";
       inputs.nixpkgs.follows = "nixpkgs";
     };
-    devenv-root = {
-      url = "file+file:///dev/null";
-      flake = false;
-    };
   };
 
   nixConfig = {
     extra-trusted-public-keys = [
-      "devenv.cachix.org-1:w1cLUi8dv3hnoSPGAuibQv+f9TZLr6cv/Hm9XgU50cw="
+      "nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs="
+      "phundrak.cachix.org-1:osJAkYO0ioTOPqaQCIXMfIRz1/+YYlVFkup3R2KSexk="
+      "cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY="
     ];
     extra-substituters = [
-      "https://devenv.cachix.org"
+      "https://phundrak.cachix.org?priority=10"
+      "https://nix-community.cachix.org?priority=20"
+      "https://cache.nixos.org?priority=30"
     ];
   };
 
   outputs = {
-    self,
     nixpkgs,
     flake-utils,
     rust-overlay,
     alejandra,
     ...
-  } @ inputs:
+  }:
     flake-utils.lib.eachDefaultSystem (
       system: let
         overlays = [(import rust-overlay)];
         pkgs = import nixpkgs {inherit system overlays;};
         rustVersion = pkgs.rust-bin.stable.latest.default;
-        rustPlatform = pkgs.makeRustPlatform {
-          cargo = rustVersion;
-          rustc = rustVersion;
+        targets = {
+          linux-x86_64 = {
+            crossPkgs = pkgs;
+            triple = "x86_64-unknown-linux-gnu";
+          };
+          linux-aarch64 = {
+            crossPkgs = pkgs.pkgsCross.aarch64-multiplatform;
+            triple = "aarch64-unknown-linux-gnu";
+          };
+        };
+        mkRustBuild = import ./nix/backend.nix;
+        packages = {
+          linux-x86_64 = mkRustBuild targets.linux-x86_64;
+          linux-aarch64 = mkRustBuild targets.linux-aarch64;
+        };
+        defaultBySystem = {
+          "x86_64-linux" = packages.linux-x86_64;
+          "aarch64-linux" = packages.linux-aarch64;
         };
       in {
         formatter = alejandra.defaultPackage.${system};
-        packages = import ./nix/package.nix {inherit pkgs rustPlatform;};
-        devShell = import ./nix/shell.nix {
-          inherit inputs pkgs self rustVersion system;
-        };
+        packages.backend =
+          packages
+          // {
+            default = defaultBySystem.${system} or packages.linux-x86_64;
+          };
+        devShell = import ./nix/shell.nix {inherit pkgs rustVersion;};
       }
     );
 }

justfile

@@ -30,6 +30,7 @@ release-build:
 release-run:
 	cargo run --release
 
+[env("SQLX_OFFLINE", "1")]
 test:
 	cargo test --all --all-targets
 

nix/backend.nix

@@ -0,0 +1,24 @@
+target: let
+  cargoToml = fromTOML (builtins.readFile ../backend/Cargo.toml);
+  inherit (cargoToml.package) name version;
+  pkgs = target.crossPkgs;
+  buildArgs = {
+    pname = name;
+    inherit version;
+    src = pkgs.lib.cleanSource ../.;
+    cargoLock.lockFile = ../Cargo.lock;
+    useNextest = true;
+    meta = {
+      inherit (cargoToml.package) description homepage;
+    };
+    postBuild = "${pkgs.upx}/bin/upx target/*/release/*${name}";
+  };
+  rustVersion = pkgs.rust-bin.stable.latest.default.override {
+    targets = [target.triple];
+  };
+  rustPlatform = target.crossPkgs.makeRustPlatform {
+    cargo = rustVersion;
+    rustc = rustVersion;
+  };
+in
+  rustPlatform.buildRustPackage buildArgs

nix/package.nix

@@ -1,21 +0,0 @@
-{
-  pkgs,
-  rustPlatform,
-  ...
-}: let
-  cargoToml = builtins.fromTOML (builtins.readFile ../Cargo.toml);
-  name = cargoToml.package.name;
-  version = cargoToml.package.version;
-  rustBuild = rustPlatform.buildRustPackage {
-    pname = name;
-    inherit version;
-    src = ../.;
-    cargoLock.lockFile = ../Cargo.lock;
-  };
-  settingsDir = pkgs.runCommand "settings" {} ''
-    mkdir -p $out/settings
-    cp ${../settings}/*.yaml $out/settings/
-  '';
-in {
-  jj-mcp = rustBuild;
-}

nix/rust-version.nix

@@ -1,11 +0,0 @@
-{
-  rust-overlay,
-  inputs,
-  system,
-  ...
-}: let
-  overlays = [(import rust-overlay)];
-in rec {
-  pkgs = import inputs.nixpkgs {inherit system overlays;};
-  version = pkgs.rust-bin.stable.latest.default;
-}

nix/shell.nix

@@ -1,58 +1,32 @@
 {
-  inputs,
   pkgs,
-  self,
   rustVersion,
-  system,
-  ...
 }:
-inputs.devenv.lib.mkShell {
-  inherit inputs pkgs;
-  modules = [
-    {
-      packages = with pkgs; [
-        # Backend
-        (rustVersion.override {
-          extensions = [
-            "clippy"
-            "rust-src"
-            "rust-analyzer"
-            "rustfmt"
-          ];
-        })
-        bacon
-        cargo-deny
-        cargo-edit
-        cargo-shuttle
-        cargo-tarpaulin
-        just
-        marksman # Markdown LSP server
-        sqlx-cli
-        tombi # TOML LSP server
-
-        # Frontend
-        nodejs_24
-        rustywind # tailwind
-        nodePackages.prettier
-        nodePackages.eslint
-        nodePackages.pnpm
+pkgs.mkShell {
+  packages = with pkgs; [
+    (rustVersion.override {
+      extensions = [
+        "clippy"
+        "rust-src"
+        "rust-analyzer"
+        "rustfmt"
       ];
+    })
+    bacon
+    cargo-deny
+    cargo-edit
+    cargo-shuttle
+    cargo-tarpaulin
+    just
+    marksman # Markdown LSP server
+    sqlx-cli
+    tombi # TOML LSP server
 
-      processes.run.exec = "bacon run";
-
-      enterShell = ''
-        echo "🦀 Rust MCP development environment loaded!"
-        echo "📦 Rust version: $(rustc --version)"
-        echo "📦 Cargo version: $(cargo --version)"
-        echo ""
-        echo "Available tools:"
-        echo "  - rust-analyzer (LSP)"
-        echo "  - clippy (linter)"
-        echo "  - rustfmt (formatter)"
-        echo "  - bacon (continuous testing/linting)"
-        echo "  - cargo-deny (dependency checker)"
-        echo "  - cargo-tarpaulin (code coverage)"
-      '';
-    }
+    # Frontend
+    nodejs_24
+    rustywind # tailwind
+    prettier
+    eslint
+    pnpm
   ];
 }

specs/001-modbus-relay-control/tasks.org

@@ -1,6 +1,7 @@
 #+title: Implementation Tasks: Modbus Relay Control System
 #+author: Lucien Cartier-Tilet
 #+email: lucien@phundrak.com
+#+startup: content align hideblocks
 #+options: ^:nil
 #+LATEX_CLASS_OPTIONS: [a4paper,10pt]
 #+LATEX_HEADER: \makeatletter \@ifpackageloaded{geometry}{\geometry{margin=2cm}}{\usepackage[margin=2cm]{geometry}} \makeatother
@@ -586,7 +587,7 @@ CLOSED: [2026-01-22 jeu. 00:02]
 
 --------------
 
-** STARTED Phase 4: US1 - Monitor & Toggle Relay States (MVP) (2 days) [1/5]
+** STARTED Phase 4: US1 - Monitor & Toggle Relay States (MVP) (2 days) [3/5]
 - State "STARTED"    from "TODO"       [2026-01-23 ven. 20:20]
 *Goal*: View current state of all 8 relays + toggle individual relay on/off
 
@@ -616,27 +617,108 @@ CLOSED: [2026-01-23 ven. 20:42]
   - *File*: =src/application/use_cases/get_all_relays.rs=
   - *Complexity*: Low | *Uncertainty*: Low
 
-*** STARTED Presentation Layer (Backend API) [0/2]
+*** DONE Presentation Layer (Backend API) [3/3]
+CLOSED: [2026-05-14 jeu. 18:43]
+- State "DONE"       from "TODO"       [2026-05-14 jeu. 18:43]
 - State "STARTED"    from "TODO"       [2026-01-23 ven. 20:42]
-- [ ] *T045* [US1] [TDD] Define =RelayDto= in presentation layer
+- [X] *T045* [US1] [TDD] Define =RelayDto= in presentation layer
   - Fields: =id= (=u8=), =state= ("on"/"off"), =label= (=Option=)
   - Implement =From= for =RelayDto=
   - *File*: =src/presentation/dto/relay_dto.rs=
   - *Complexity*: Low | *Uncertainty*: Low
-- [ ] *T046* [US1] [TDD] Define API error responses
+- [X] *T046* [US1] [TDD] Define API error responses
   - =ApiError= enum with status codes and messages
   - Implement =poem::error::ResponseError=
   - *File*: =src/presentation/error.rs=
   - *Complexity*: Low | *Uncertainty*: Low
+- [X] *T047* [US1] [TDD] Create =RelayApi= struct with dependency injection
+  - Create =RelayApi= struct that holds dependencies:
+    - =relay_controller: Arc<dyn RelayController>=
+    - =label_repository: Arc<dyn RelayLabelRepository>=
+  - Implement constructor: =RelayApi::new(controller, repository) -> Self=
+  - Add =#[derive(Clone)]= to allow sharing across poem-openapi
+  - *File*: =src/presentation/api/relay_api.rs= or =src/route/relay.rs=
+  - *Complexity*: Low | *Uncertainty*: Low
+
+  *TDD Checklist*:
+
+  - [ ] Test: =RelayApi::new()= creates instance with provided dependencies
+  - [ ] Test: =RelayApi= can be cloned (required for poem-openapi)
+  - [ ] Test: Constructor stores both controller and repository
+
+  *Pseudocode*:
+
+  #+begin_src rust
+  use std::sync::Arc;
+  use crate::domain::relay::{
+      controller::RelayController,
+      repository::RelayLabelRepository,
+  };
+
+  /// API handler for relay control endpoints.
+  ///
+  /// This struct holds the dependencies needed for relay operations
+  /// and implements the poem-openapi handlers.
+  #[derive(Clone)]
+  pub struct RelayApi {
+      relay_controller: Arc<dyn RelayController>,
+      label_repository: Arc<dyn RelayLabelRepository>,
+  }
+
+  impl RelayApi {
+      /// Creates a new RelayApi with the provided dependencies.
+      ///
+      /// # Arguments
+      ///
+      /// * `relay_controller` - Controller for reading/writing relay states
+      /// * `label_repository` - Repository for managing relay labels
+      pub fn new(
+          relay_controller: Arc<dyn RelayController>,
+          label_repository: Arc<dyn RelayLabelRepository>,
+      ) -> Self {
+          Self {
+              relay_controller,
+              label_repository,
+          }
+      }
+  }6 lerolero 7
+
+  #[cfg(test)]
+  mod tests {
+      use super::*;
+      use crate::infrastructure::modbus::MockRelayController;
+      use crate::infrastructure::persistence::MockLabelRepository;
+
+      #[test]
+      fn test_relay_api_new_creates_instance() {
+          // GIVEN: Mock dependencies
+          let controller = Arc::new(MockRelayController::new());
+          let repository = Arc::new(MockLabelRepository::new());
+
+          // WHEN: Creating RelayApi
+          let api = RelayApi::new(controller.clone(), repository.clone());
+
+          // THEN: Instance is created successfully
+          // Verify by checking that we can clone it (required for poem-openapi)
+          let _cloned_api = api.clone();
+      }
+  }
+  #+end_src
+
+  *Note*: After this task, T048-T051 will add endpoint methods to this struct.
 
 --------------
 
-*** TODO T039: Dependency Injection Setup (DECOMPOSED) [0/8]
+*** DONE T039: Dependency Injection Setup (DECOMPOSED) [8/8]
+CLOSED: [2026-05-14 jeu. 20:09]
+- State "DONE"       from "STARTED"    [2026-05-14 jeu. 20:09]
+- State "STARTED"    from "TODO"       [2026-03-06 ven. 22:11]
 - Complexity :: High → Broken into 4 sub-tasks
 - Uncertainty :: Medium
 - Rationale :: Graceful degradation (FR-023), conditional mock/real controller
+- Prerequisites :: T047 (RelayApi struct) must be complete before T039c
 
-- [ ] *T039a* [US1] [TDD] Create =ModbusRelayController= factory with retry and fallback
+- [X] *T039a* [US1] [TDD] Create =ModbusRelayController= factory with retry and fallback
 
   - Factory function: ~create_relay_controller(settings, use_mock) => Arc~
   - Retry 3 times with 2s backoff on connection failure
@@ -692,13 +774,12 @@ CLOSED: [2026-01-23 ven. 20:42]
 
   *TDD Checklist*:
 
-  - [ ] Test: use_mock=true returns =MockRelayController= immediately
-  - [ ] Test: Successful connection returns =ModbusRelayController=
-  - [ ] Test: Connection failure after 3 retries returns =MockRelayController=
-  - [ ] Test: Retry delays are 2 seconds between attempts
-  - [ ] Test: Logs appropriate messages for each connection attempt
-
-- [ ] *T039b* [US4] [TDD] Create =RelayLabelRepositor=y factory
+  - [X] Test: ~use_mock=true~ returns =MockRelayController= immediately
+  - [X] Test: Successful connection returns =ModbusRelayController=
+  - [X] Test: Connection failure after 3 retries returns =MockRelayController=
+  - [X] Test: Retry delays are 2 seconds between attempts
+  - [X] Test: Logs appropriate messages for each connection attempt
+- [X] *T039b* [US4] [TDD] Create =RelayLabelRepository= factory
 
   - Factory function: ~create_label_repository(db_path, use_mock) => Arc~
   - If use_mock: return =MockLabelRepository=
@@ -725,17 +806,19 @@ CLOSED: [2026-01-23 ven. 20:42]
 
   *TDD Checklist*:
 
-  - [ ] Test: use_mock=true returns =MockLabelRepository=
-  - [ ] Test: use_mock=false returns =SQLiteLabelRepository=
-  - [ ] Test: Invalid =db_path= returns =RepositoryError=
-
-- [ ] *T039c* [US1] [TDD] Wire dependencies in =Application::build()=
+  - [X] Test: use_mock=true returns =MockLabelRepository=
+  - [X] Test: use_mock=false returns =SQLiteLabelRepository=
+  - [X] Test: Invalid =db_path= returns =RepositoryError=
+- [X] *T039c* [US1] [TDD] Wire dependencies in =Application::build()=
 
+  - *Prerequisites*: T047 must be complete (RelayApi struct created)
   - Determine test mode: ~cfg!(test) || env::var("CI").is_ok()~
   - Call =create_relay_controller()= and =create_label_repository()=
-  - Pass dependencies to =RelayApi::new()=
+  - Create =RelayApi= instance with dependencies (requires T047)
+  - Pass =RelayApi= to OpenAPI service
   - *File*: =src/startup.rs=
   - *Complexity*: Medium | *Uncertainty*: Low
+  - *Note*: Tests for T039c have been written (they currently pass trivially)
 
   *Pseudocode*:
 
@@ -770,12 +853,10 @@ CLOSED: [2026-01-23 ven. 20:42]
 
   *TDD Checklist*:
 
-  - [ ] Test: =Application::build()= succeeds in test mode
-  - [ ] Test: =Application::build()= creates correct mock dependencies when CI=true
-  - [ ] Test: =Application::build()= creates real dependencies when not in test mode
-
-- [ ] *T039d* [US1] [TDD] Register =RelayApi= in route aggregator
-
+  - [X] Test: =Application::build()= succeeds in test mode
+  - [X] Test: =Application::build()= creates correct mock dependencies when CI=true
+  - [X] Test: =Application::build()= creates real dependencies when not in test mode
+- [X] *T039d* [US1] [TDD] Register =RelayApi= in route aggregator
   - Add =RelayApi= to OpenAPI service
   - Tag: "Relays"
   - *File*: =src/startup.rs=
@@ -783,28 +864,28 @@ CLOSED: [2026-01-23 ven. 20:42]
 
   *TDD Checklist*:
 
-  - [ ] Test: OpenAPI spec includes =/api/relays= endpoints
-  - [ ] Test: Swagger UI renders =Relays= tag
+  - [X] Test: OpenAPI spec includes =/api/relays= endpoints
+  - [X] Test: Swagger UI renders =Relays= tag
 
 --------------
 
-- [ ] *T048* [US1] [TDD] Write contract tests for =GET /api/relays=
+- [X] *T048* [US1] [TDD] Write contract tests for =GET /api/relays=
   - Test: Returns 200 with array of 8 =RelayDto=
   - Test: Each relay has id 1-8, state, and optional label
   - *File*: =tests/contract/test_relay_api.rs=
   - *Complexity*: Low | *Uncertainty*: Low
-- [ ] *T049* [US1] [TDD] Implement =GET /api/relays= endpoint
+- [X] *T049* [US1] [TDD] Implement =GET /api/relays= endpoint
   - ~#[oai(path = "/relays", method = "get")]~
   - Call =GetAllRelaysUseCase=, map to =RelayDto=
   - *File*: =src/presentation/api/relay_api.rs=
   - *Complexity*: Low | *Uncertainty*: Low
-- [ ] *T050* [US1] [TDD] Write contract tests for =POST /api/relays/{id}/toggle=
+- [X] *T050* [US1] [TDD] Write contract tests for =POST /api/relays/{id}/toggle=
   - Test: Returns 200 with updated =RelayDto=
   - Test: Returns 404 for id < 1 or id > 8
   - Test: State actually changes in controller
   - *File*: =tests/contract/test_relay_api.rs=
   - *Complexity*: Low | *Uncertainty*: Low
-- [ ] *T051* [US1] [TDD] Implement =POST /api/relays/{id}/toggle= endpoint
+- [X] *T051* [US1] [TDD] Implement =POST /api/relays/{id}/toggle= endpoint
   - ~#[oai(path = "/relays/:id/toggle", method = "post")]~
   - Parse id, call =ToggleRelayUseCase=, return updated state
   - *File*: =src/presentation/api/relay_api.rs=