test(infrastructure): implement MockRelayLabelRepository for testing

Create in-memory mock implementation of RelayLabelRepository trait
using HashMap with Arc<Mutex<>> for thread-safe concurrent access.
Includes 8 comprehensive tests covering all trait methods and edge
cases.

Also refactor domain repository module structure to support multiple
repository types (repository.rs → repository/label.rs + mod.rs).

TDD phase: Combined red-green (tests + implementation)

Ref: T037, T038

backend/src/domain/relay/repository/label.rs

@@ -1,20 +1,8 @@
 use async_trait::async_trait;
 
-use super::types::{RelayId, RelayLabel};
+use crate::domain::relay::types::{RelayId, RelayLabel};
 
-/// Errors that can occur during repository operations.
-///
-/// This enum provides structured error handling for all data persistence
-/// operations related to relay management.
-#[derive(Debug, thiserror::Error)]
-pub enum RepositoryError {
-    /// A database operation failed with the given error message.
-    #[error("Database error: {0}")]
-    DatabaseError(String),
-    /// The requested relay was not found in the repository.
-    #[error("Relay not found: {0}")]
-    NotFound(RelayId),
-}
+use super::RepositoryError;
 
 /// Repository trait for persisting and retrieving relay labels.
 ///

backend/src/domain/relay/repository/mod.rs

@@ -0,0 +1,18 @@
+mod label;
+pub use label::RelayLabelRepository;
+
+use super::types::RelayId;
+
+/// Errors that can occur during repository operations.
+///
+/// This enum provides structured error handling for all data persistence
+/// operations related to relay management.
+#[derive(Debug, thiserror::Error)]
+pub enum RepositoryError {
+    /// A database operation failed with the given error message.
+    #[error("Database error: {0}")]
+    DatabaseError(String),
+    /// The requested relay was not found in the repository.
+    #[error("Relay not found: {0}")]
+    NotFound(RelayId),
+}

backend/src/infrastructure/persistence/label_repository.rs

@@ -0,0 +1,237 @@
+//! Mock implementation and tests for `RelayLabelRepository` trait.
+//!
+//! This module provides a simple in-memory mock implementation of the
+//! `RelayLabelRepository` trait for testing purposes, along with comprehensive
+//! tests that verify the trait's contract.
+
+use std::collections::HashMap;
+use std::sync::Arc;
+
+use async_trait::async_trait;
+use tokio::sync::{Mutex, MutexGuard};
+
+use crate::domain::relay::{
+    repository::{RelayLabelRepository, RepositoryError},
+    types::{RelayId, RelayLabel},
+};
+
+/// In-memory mock implementation of `RelayLabelRepository` for testing.
+///
+/// This implementation uses a `HashMap` wrapped in `Arc<Mutex<_>>` to provide
+/// thread-safe concurrent access to relay labels. It's useful for testing
+/// application logic without requiring a database connection.
+#[derive(Debug, Clone)]
+pub struct MockRelayLabelRepository {
+    /// Internal storage for relay labels, protected by a mutex for thread safety.
+    labels: Arc<Mutex<HashMap<u8, RelayLabel>>>,
+}
+
+impl MockRelayLabelRepository {
+    /// Creates a new empty mock repository.
+    #[must_use]
+    pub fn new() -> Self {
+        Self {
+            labels: Arc::new(Mutex::new(HashMap::new())),
+        }
+    }
+
+    async fn labels(&self) -> MutexGuard<'_, HashMap<u8, RelayLabel>> {
+        self.labels.lock().await
+    }
+}
+
+impl Default for MockRelayLabelRepository {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+#[async_trait]
+impl RelayLabelRepository for MockRelayLabelRepository {
+    async fn get_label(&self, id: RelayId) -> Result<Option<RelayLabel>, RepositoryError> {
+        Ok(self.labels().await.get(&id.as_u8()).cloned())
+    }
+
+    async fn save_label(&self, id: RelayId, label: RelayLabel) -> Result<(), RepositoryError> {
+        self.labels().await.insert(id.as_u8(), label);
+        Ok(())
+    }
+
+    async fn get_all_labels(&self) -> Result<Vec<(RelayId, RelayLabel)>, RepositoryError> {
+        let mut result: Vec<(RelayId, RelayLabel)> = self
+            .labels()
+            .await
+            .iter()
+            .filter_map(|(&id, label)| {
+                RelayId::new(id).map_or(None, |relay_id| Some((relay_id, label.clone())))
+            })
+            .collect();
+
+        // Sort by relay ID for consistent ordering
+        result.sort_by_key(|(id, _)| id.as_u8());
+
+        Ok(result)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[tokio::test]
+    async fn test_get_label_returns_none_for_non_existent_relay() {
+        // Test: get_label(RelayId(1)) → None when no label is set
+        let repo = MockRelayLabelRepository::new();
+        let relay_id = RelayId::new(1).unwrap();
+
+        let result = repo.get_label(relay_id).await;
+
+        assert!(result.is_ok());
+        assert!(result.unwrap().is_none());
+    }
+
+    #[tokio::test]
+    async fn test_save_label_and_get_label_returns_saved_label() {
+        // Test: save_label(RelayId(1), "Pump") then get_label(RelayId(1)) → Some("Pump")
+        let repo = MockRelayLabelRepository::new();
+        let relay_id = RelayId::new(1).unwrap();
+        let label = RelayLabel::new("Pump".to_string()).unwrap();
+
+        // Save the label
+        let save_result = repo.save_label(relay_id, label.clone()).await;
+        assert!(save_result.is_ok());
+
+        // Retrieve the label
+        let get_result = repo.get_label(relay_id).await;
+        assert!(get_result.is_ok());
+
+        let retrieved_label = get_result.unwrap();
+        assert!(retrieved_label.is_some());
+        assert_eq!(retrieved_label.unwrap().as_str(), "Pump");
+    }
+
+    #[tokio::test]
+    async fn test_save_label_overwrites_existing_label() {
+        // Test: save_label twice with different values, get_label returns the latest
+        let repo = MockRelayLabelRepository::new();
+        let relay_id = RelayId::new(1).unwrap();
+        let label1 = RelayLabel::new("First".to_string()).unwrap();
+        let label2 = RelayLabel::new("Second".to_string()).unwrap();
+
+        // Save first label
+        repo.save_label(relay_id, label1).await.unwrap();
+
+        // Overwrite with second label
+        repo.save_label(relay_id, label2).await.unwrap();
+
+        // Retrieve should return the second label
+        let retrieved_label = repo.get_label(relay_id).await.unwrap();
+        assert!(retrieved_label.is_some());
+        assert_eq!(retrieved_label.unwrap().as_str(), "Second");
+    }
+
+    #[tokio::test]
+    async fn test_get_all_labels_returns_empty_vec_when_no_labels() {
+        // Test: get_all_labels() → [] when repository is empty
+        let repo = MockRelayLabelRepository::new();
+
+        let result = repo.get_all_labels().await;
+
+        assert!(result.is_ok());
+        assert!(result.unwrap().is_empty());
+    }
+
+    #[tokio::test]
+    async fn test_get_all_labels_returns_all_saved_labels() {
+        // Test: save multiple labels, get_all_labels() returns all of them
+        let repo = MockRelayLabelRepository::new();
+
+        let relay1 = RelayId::new(1).unwrap();
+        let relay3 = RelayId::new(3).unwrap();
+        let relay5 = RelayId::new(5).unwrap();
+
+        let label1 = RelayLabel::new("Pump".to_string()).unwrap();
+        let label3 = RelayLabel::new("Heater".to_string()).unwrap();
+        let label5 = RelayLabel::new("Fan".to_string()).unwrap();
+
+        // Save labels
+        repo.save_label(relay1, label1.clone()).await.unwrap();
+        repo.save_label(relay3, label3.clone()).await.unwrap();
+        repo.save_label(relay5, label5.clone()).await.unwrap();
+
+        // Retrieve all labels
+        let result = repo.get_all_labels().await.unwrap();
+
+        assert_eq!(result.len(), 3);
+
+        // Verify labels are sorted by relay ID
+        assert_eq!(result[0].0.as_u8(), 1);
+        assert_eq!(result[0].1.as_str(), "Pump");
+
+        assert_eq!(result[1].0.as_u8(), 3);
+        assert_eq!(result[1].1.as_str(), "Heater");
+
+        assert_eq!(result[2].0.as_u8(), 5);
+        assert_eq!(result[2].1.as_str(), "Fan");
+    }
+
+    #[tokio::test]
+    async fn test_get_all_labels_excludes_relays_without_labels() {
+        // Test: Only relays with labels are returned, not all possible relay IDs
+        let repo = MockRelayLabelRepository::new();
+
+        let relay2 = RelayId::new(2).unwrap();
+        let label2 = RelayLabel::new("Only This One".to_string()).unwrap();
+
+        repo.save_label(relay2, label2).await.unwrap();
+
+        let result = repo.get_all_labels().await.unwrap();
+
+        assert_eq!(result.len(), 1);
+        assert_eq!(result[0].0.as_u8(), 2);
+    }
+
+    #[tokio::test]
+    async fn test_save_label_for_all_valid_relay_ids() {
+        // Test: All relay IDs (1-8) can have labels saved
+        let repo = MockRelayLabelRepository::new();
+
+        for id in 1..=8 {
+            let relay_id = RelayId::new(id).unwrap();
+            let label = RelayLabel::new(format!("Relay {id}")).unwrap();
+
+            let result = repo.save_label(relay_id, label).await;
+            assert!(result.is_ok(), "Failed to save label for relay {id}");
+        }
+
+        // Verify all labels were saved
+        let all_labels = repo.get_all_labels().await.unwrap();
+        assert_eq!(all_labels.len(), 8);
+    }
+
+    #[tokio::test]
+    async fn test_concurrent_access_is_thread_safe() {
+        // Test: Multiple concurrent operations don't cause data races
+        let repo = MockRelayLabelRepository::new();
+
+        let handles: Vec<_> = (1..=8)
+            .map(|id| {
+                let repo = repo.clone();
+                tokio::spawn(async move {
+                    let relay_id = RelayId::new(id).unwrap();
+                    let label = RelayLabel::new(format!("Relay {id}")).unwrap();
+                    repo.save_label(relay_id, label).await.unwrap();
+                })
+            })
+            .collect();
+
+        // Wait for all tasks to complete
+        for handle in handles {
+            handle.await.unwrap();
+        }
+
+        // Verify all labels were saved correctly
+        let all_labels = repo.get_all_labels().await.unwrap();
+        assert_eq!(all_labels.len(), 8);
+    }
+}

backend/src/infrastructure/persistence/mod.rs

@@ -3,5 +3,8 @@
 //! This module contains the concrete implementations of repository traits
 //! for data persistence, including SQLite-based storage for relay labels.
 
+/// Mock repository implementation for testing.
+pub mod label_repository;
+
 /// `SQLite` repository implementation for relay labels.
 pub mod sqlite_repository;

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

@@ -76,7 +76,7 @@
   - Test: CorsSettings with wildcard origin deserializes correctly ✓
   - Test: Settings::new() loads cors section from development.yaml ✓
   - Test: CorsSettings with partial fields uses defaults ✓
-  - **File**: backend/src/settings.rs (in tests module)
+  - **File**: `backend/src/settings.rs (in tests module)`
   - **Complexity**: Low | **Uncertainty**: Low
   - **Tests Written**: 5 tests (cors_settings_deserialize_from_yaml, cors_settings_default_has_empty_origins, cors_settings_with_wildcard_deserializes, settings_loads_cors_section_from_yaml, cors_settings_deserialize_with_defaults)
 
@@ -86,14 +86,14 @@
   - Add `#[derive(Debug, serde::Deserialize, Clone)]` to struct
   - Add `#[serde(default)]` attribute to Settings.cors field
   - Update Settings struct to include `pub cors: CorsSettings`
-  - **File**: backend/src/settings.rs
+  - **File**: `backend/src/settings.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [x] **T011** [Setup] [TDD] Update development.yaml with permissive CORS settings
   - Add cors section with `allowed_origins: ["*"]`, `allow_credentials: false`, `max_age_secs: 3600`
   - Update `frontend_url` from `http://localhost:3000` to `http://localhost:5173` (Vite default port)
   - **Test**: cargo run loads development config without errors
-  - **File**: backend/settings/development.yaml
+  - **File**: `backend/settings/development.yaml`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [x] **T012** [P] [Setup] [TDD] Create production.yaml with restrictive CORS settings
@@ -101,7 +101,7 @@
   - Add `frontend_url: "https://REDACTED"`
   - Add production-specific application settings (protocol: https, host: 0.0.0.0)
   - **Test**: Settings::new() with APP_ENVIRONMENT=production loads config
-  - **File**: backend/settings/production.yaml
+  - **File**: `backend/settings/production.yaml`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [x] **T013** [Setup] [TDD] Write tests for build_cors() function
@@ -111,7 +111,7 @@
   - Test: build_cors() sets correct methods (GET, POST, PUT, PATCH, DELETE, OPTIONS) ✓
   - Test: build_cors() sets correct headers (content-type, authorization) ✓
   - Test: build_cors() sets max_age from settings ✓
-  - **File**: backend/src/startup.rs (in tests module)
+  - **File**: `backend/src/startup.rs (in tests module)`
   - **Complexity**: Medium | **Uncertainty**: Low
 
 - [x] **T014** [Setup] [TDD] Implement build_cors() free function in startup.rs
@@ -123,7 +123,7 @@
   - Set `allow_credentials` from settings
   - Set `max_age` from settings.max_age_secs
   - Add structured logging: `tracing::info!(allowed_origins = ?settings.allowed_origins, allow_credentials = settings.allow_credentials, "CORS middleware configured")`
-  - **File**: backend/src/startup.rs
+  - **File**: `backend/src/startup.rs`
   - **Complexity**: Medium | **Uncertainty**: Low
 
   **Pseudocode**:
@@ -174,7 +174,7 @@
   - In `From<Application> for RunnableApplication`, replace `.with(Cors::new())` with `.with(Cors::from(value.settings.cors.clone()))` ✓
   - CORS is applied after rate limiting (order: RateLimit → CORS → Data) ✓
   - **Test**: Unit test verifies CORS middleware uses settings ✓
-  - **File**: backend/src/startup.rs (line 84)
+  - **File**: `backend/src/startup.rs (line 84)`
   - **Complexity**: Low | **Uncertainty**: Low
   - **Note**: Used `From<CorsSettings> for Cors` trait instead of `build_cors()` function (better design pattern)
 
@@ -188,7 +188,7 @@
   - Test: Multiple origins are supported ✓
   - Test: Unauthorized origins are rejected with 403 ✓
   - Test: Credentials disabled by default ✓
-  - **File**: backend/tests/cors_test.rs (9 integration tests)
+  - **File**: `backend/tests/cors_test.rs (9 integration tests)`
   - **Complexity**: Medium | **Uncertainty**: Low
   - **Tests Written**: 9 comprehensive integration tests covering all CORS scenarios
 
@@ -208,27 +208,27 @@
   - Test: RelayId::new(0) → Err(InvalidRelayId)
   - Test: RelayId::new(9) → Err(InvalidRelayId)
   - Test: RelayId::as_u8() returns inner value
-  - **File**: src/domain/relay.rs
+  - **File**: `src/domain/relay.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [x] **T018** [US1] [TDD] Implement RelayId newtype with validation
   - #[repr(transparent)] newtype wrapping u8
   - Constructor validates 1..=8 range
   - Implement Display, Debug, Clone, Copy, PartialEq, Eq
-  - **File**: src/domain/relay.rs
+  - **File**: `src/domain/relay.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [x] **T019** [P] [US1] [TDD] Write tests for RelayState enum
   - Test: RelayState::On → serializes to "on"
   - Test: RelayState::Off → serializes to "off"
   - Test: Parse "on"/"off" from strings
-  - **File**: src/domain/relay.rs
+  - **File**: `src/domain/relay.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [x] **T020** [P] [US1] [TDD] Implement RelayState enum
   - Enum: On, Off
   - Implement Display, Debug, Clone, Copy, PartialEq, Eq, serde::Serialize/Deserialize
-  - **File**: src/domain/relay.rs
+  - **File**: `src/domain/relay.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [x] **T021** [US1] [TDD] Write tests for Relay aggregate
@@ -236,13 +236,13 @@
   - Test: relay.toggle() flips state
   - Test: relay.turn_on() sets state to On
   - Test: relay.turn_off() sets state to Off
-  - **File**: src/domain/relay.rs
+  - **File**: `src/domain/relay.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [x] **T022** [US1] [TDD] Implement Relay aggregate
   - Struct: `Relay { id: RelayId, state: RelayState, label: Option<RelayLabel> }`
   - Methods: `new()` `toggle()` `turn_on()` `turn_off()` `state()` `label()`
-  - **File**: src/domain/relay.rs
+  - **File**: `src/domain/relay.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [x] **T023** [P] [US4] [TDD] Write tests for RelayLabel newtype
@@ -250,32 +250,32 @@
   - Test: RelayLabel::new("A".repeat(50)) → Ok
   - Test: RelayLabel::new("") → Err(EmptyLabel)
   - Test: RelayLabel::new("A".repeat(51)) → Err(LabelTooLong)
-  - **File**: src/domain/relay.rs
+  - **File**: `src/domain/relay.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [x] **T024** [P] [US4] [TDD] Implement RelayLabel newtype
   - #[repr(transparent)] newtype wrapping String
   - Constructor validates 1..=50 length
   - Implement Display, Debug, Clone, PartialEq, Eq
-  - **File**: src/domain/relay.rs
+  - **File**: `src/domain/relay.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [x] **T025** [US1] [TDD] Write tests for ModbusAddress type
   - Test: ModbusAddress::from(RelayId(1)) → ModbusAddress(0)
   - Test: ModbusAddress::from(RelayId(8)) → ModbusAddress(7)
-  - **File**: src/domain/modbus.rs
+  - **File**: `src/domain/modbus.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [x] **T026** [US1] [TDD] Implement ModbusAddress type with From<RelayId>
   - #[repr(transparent)] newtype wrapping u16
   - Implement From<RelayId> with offset: user 1-8 → Modbus 0-7
-  - **File**: src/domain/modbus.rs
+  - **File**: `src/domain/modbus.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [x] **T027** [US3] [TDD] Write tests and implement HealthStatus enum
   - Enum: Healthy, Degraded { consecutive_errors: u32 }, Unhealthy { reason: String }
   - Test transitions between states
-  - **File**: src/domain/health.rs
+  - **File**: `src/domain/health.rs`
   - **Complexity**: Medium | **Uncertainty**: Low
 
 **Checkpoint**: Domain types complete with 100% test coverage
@@ -290,13 +290,13 @@
   - Test: read_state() returns mocked state
   - Test: write_state() updates mocked state
   - Test: read_all() returns 8 relays in known state
-  - **File**: src/infrastructure/modbus/mock_controller.rs
+  - **File**: `src/infrastructure/modbus/mock_controller.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [x] **T029** [P] [US1] [TDD] Implement MockRelayController
   - Struct with Arc<Mutex<HashMap<RelayId, RelayState>>>
   - Implement RelayController trait with in-memory state
-  - **File**: src/infrastructure/modbus/mock_controller.rs
+  - **File**: `src/infrastructure/modbus/mock_controller.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [x] **T030** [US1] [TDD] Define RelayController trait
@@ -304,14 +304,14 @@
   - async fn write_state(&self, id: RelayId, state: RelayState) → Result<(), ControllerError>
   - async fn read_all(&self) → Result<Vec<(RelayId, RelayState)>, ControllerError>
   - async fn write_all(&self, state: RelayState) → Result<(), ControllerError>
-  - **File**: src/infrastructure/modbus/controller.rs
+  - **File**: `src/infrastructure/modbus/controller.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [x] **T031** [P] [US1] [TDD] Define ControllerError enum
   - Variants: ConnectionError(String), Timeout(u64), ModbusException(String), InvalidRelayId(u8)
   - Implement std::error::Error, Display, Debug
   - Use thiserror derive macros
-  - **File**: src/infrastructure/modbus/error.rs
+  - **File**: `src/infrastructure/modbus/error.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [x] **T032** [US1] [TDD] Write tests for MockRelayController
@@ -321,7 +321,7 @@
   - Test: write_relay_state() for all 8 relays independently ✓
   - Test: read_relay_state() with invalid relay ID (type system prevents) ✓
   - Test: concurrent access is thread-safe ✓
-  - **File**: src/infrastructure/modbus/mock_controller.rs
+  - **File**: `src/infrastructure/modbus/mock_controller.rs`
   - **Complexity**: Low | **Uncertainty**: Low
   - **Tests Written**: 6 comprehensive tests covering all mock controller scenarios
 
@@ -338,7 +338,7 @@
   - Struct: `ModbusRelayController { ctx: Arc<Mutex<Context>>, timeout_duration: Duration }`
   - Constructor: `new(host, port, slave_id, timeout_secs) → Result<Self, ControllerError>`
   - Use `tokio_modbus::client::tcp::connect_slave()`
-  - **File**: src/infrastructure/modbus/modbus_controller.rs
+  - **File**: `src/infrastructure/modbus/modbus_controller.rs`
   - **Complexity**: Medium | **Uncertainty**: Medium
 
   **Pseudocode**:
@@ -381,7 +381,7 @@
   - Wrap `ctx.read_coils()` with `tokio::time::timeout()`
   - Handle nested Result: timeout → io::Error → Modbus Exception
   - **Note**: Modbus TCP uses MBAP header (no CRC validation needed)
-  - **File**: src/infrastructure/modbus/modbus_controller.rs
+  - **File**: `src/infrastructure/modbus/modbus_controller.rs`
   - **Complexity**: Medium | **Uncertainty**: Medium
 
   **Pseudocode** (CRITICAL PATTERN):
@@ -415,7 +415,7 @@
 - [x] **T025c** [US1] [TDD] Implement timeout-wrapped `write_single_coil` helper
   - Private method: `write_single_coil_with_timeout(addr: u16, value: bool) → Result<(), ControllerError>`
   - Similar nested Result handling as T025b
-  - **File**: src/infrastructure/modbus/modbus_controller.rs
+  - **File**: `src/infrastructure/modbus/modbus_controller.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
   **Pseudocode**:
@@ -446,7 +446,7 @@
   - Convert RelayId → ModbusAddress (0-based)
   - Call `read_coils_with_timeout(addr, 1)`
   - Convert bool → RelayState
-  - **File**: src/infrastructure/modbus/modbus_controller.rs
+  - **File**: `src/infrastructure/modbus/modbus_controller.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
   **Pseudocode**:
@@ -471,7 +471,7 @@
   - Convert RelayId → ModbusAddress
   - Convert RelayState → bool (On=true, Off=false)
   - Call `write_single_coil_with_timeout()`
-  - **File**: src/infrastructure/modbus/modbus_controller.rs
+  - **File**: `src/infrastructure/modbus/modbus_controller.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
   **Pseudocode**:
@@ -491,7 +491,7 @@
   - `read_all()`: Call `read_coils_with_timeout(0, 8)`, map to `Vec<(RelayId, RelayState)>`
   - `write_all()`: Loop over RelayId 1-8, call `write_state()` for each
   - Add `firmware_version()` method (read holding register 0x9999, optional)
-  - **File**: src/infrastructure/modbus/modbus_controller.rs
+  - **File**: `src/infrastructure/modbus/modbus_controller.rs`
   - **Complexity**: Medium | **Uncertainty**: Low
 
   **Pseudocode**:
@@ -527,42 +527,42 @@
 - [ ] **T034** [US1] [TDD] Integration test with real hardware (optional)
   - **REQUIRES PHYSICAL DEVICE**: Test against actual Modbus relay at configured IP
   - Skip if device unavailable, rely on MockRelayController for CI
-  - **File**: tests/integration/modbus_hardware_test.rs
+  - **File**: `tests/integration/modbus_hardware_test.rs`
   - **Complexity**: Medium | **Uncertainty**: High
-  - **Note**: Use #[ignore] attribute, run with cargo test -- --ignored
+  - **Note**: Use #[ignore] attribute, run with `cargo test -- --ignored`
 
 - [ ] **T035** [P] [US4] [TDD] Write tests for RelayLabelRepository trait
-  - Test: get_label(RelayId(1)) → Option<RelayLabel>
-  - Test: set_label(RelayId(1), label) → Result<(), RepositoryError>
-  - Test: delete_label(RelayId(1)) → Result<(), RepositoryError>
-  - **File**: src/infrastructure/persistence/label_repository.rs
+  - Test: `get_label(RelayId(1)) → Option<RelayLabel>`
+  - Test: `set_label(RelayId(1), label) → Result<(), RepositoryError>`
+  - Test: `delete_label(RelayId(1)) → Result<(), RepositoryError>`
+  - **File**: `src/infrastructure/persistence/label_repository.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [ ] **T036** [P] [US4] [TDD] Implement SQLite RelayLabelRepository
-  - Implement get_label(), set_label(), delete_label() using SQLx
-  - Use sqlx::query! macros for compile-time SQL verification
-  - **File**: src/infrastructure/persistence/sqlite_label_repository.rs
+  - Implement `get_label()`, `set_label()`, `delete_label()` using SQLx
+  - Use `sqlx::query!` macros for compile-time SQL verification
+  - **File**: `src/infrastructure/persistence/sqlite_label_repository.rs`
   - **Complexity**: Medium | **Uncertainty**: Low
 
-- [ ] **T037** [US4] [TDD] Write tests for in-memory mock LabelRepository
+- [x] **T037** [US4] [TDD] Write tests for in-memory mock LabelRepository
   - For testing without SQLite dependency
-  - **File**: src/infrastructure/persistence/mock_label_repository.rs
+  - **File**: `src/infrastructure/persistence/mock_label_repository.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
-- [ ] **T038** [US4] [TDD] Implement in-memory mock LabelRepository
+- [x] **T038** [US4] [TDD] Implement in-memory mock LabelRepository
   - HashMap-based implementation
-  - **File**: src/infrastructure/persistence/mock_label_repository.rs
+  - **File**: `src/infrastructure/persistence/mock_label_repository.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [ ] **T039** [US3] [TDD] Write tests for HealthMonitor service
-  - Test: track_success() transitions Degraded → Healthy
-  - Test: track_failure() transitions Healthy → Degraded → Unhealthy
-  - **File**: src/application/health_monitor.rs
+  - Test: `track_success()` transitions Degraded → Healthy
+  - Test: `track_failure()` transitions Healthy → Degraded → Unhealthy
+  - **File**: `src/application/health_monitor.rs`
   - **Complexity**: Medium | **Uncertainty**: Low
 
 - [ ] **T040** [US3] [TDD] Implement HealthMonitor service
   - Track consecutive errors, transition states per FR-020, FR-021
-  - **File**: src/application/health_monitor.rs
+  - **File**: `src/application/health_monitor.rs`
   - **Complexity**: Medium | **Uncertainty**: Low
 
 **Checkpoint**: Infrastructure layer complete with trait abstractions
@@ -580,22 +580,22 @@
 - [ ] **T041** [US1] [TDD] Write tests for ToggleRelayUseCase
   - Test: execute(RelayId(1)) toggles relay state via controller
   - Test: execute() returns error if controller fails
-  - **File**: src/application/use_cases/toggle_relay.rs
+  - **File**: `src/application/use_cases/toggle_relay.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [ ] **T042** [US1] [TDD] Implement ToggleRelayUseCase
   - Orchestrate: read current state → toggle → write new state
-  - **File**: src/application/use_cases/toggle_relay.rs
+  - **File**: `src/application/use_cases/toggle_relay.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [ ] **T043** [P] [US1] [TDD] Write tests for GetAllRelaysUseCase
   - Test: execute() returns all 8 relays with states
-  - **File**: src/application/use_cases/get_all_relays.rs
+  - **File**: `src/application/use_cases/get_all_relays.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [ ] **T044** [P] [US1] [TDD] Implement GetAllRelaysUseCase
   - Call controller.read_all(), map to domain Relay objects
-  - **File**: src/application/use_cases/get_all_relays.rs
+  - **File**: `src/application/use_cases/get_all_relays.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 ### Presentation Layer (Backend API)
@@ -603,13 +603,13 @@
 - [ ] **T045** [US1] [TDD] Define RelayDto in presentation layer
   - Fields: id (u8), state ("on"/"off"), label (Option<String>)
   - Implement From<Relay> for RelayDto
-  - **File**: src/presentation/dto/relay_dto.rs
+  - **File**: `src/presentation/dto/relay_dto.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [ ] **T046** [US1] [TDD] Define API error responses
   - ApiError enum with status codes and messages
   - Implement poem::error::ResponseError
-  - **File**: src/presentation/error.rs
+  - **File**: `src/presentation/error.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 ---
@@ -624,7 +624,7 @@
   - Factory function: create_relay_controller(settings, use_mock) → Arc<dyn RelayController>
   - Retry 3 times with 2s backoff on connection failure
   - Graceful degradation: fallback to MockRelayController if all retries fail (FR-023)
-  - **File**: src/infrastructure/modbus/factory.rs
+  - **File**: `src/infrastructure/modbus/factory.rs`
   - **Complexity**: Medium | **Uncertainty**: Medium
 
   **Pseudocode**:
@@ -683,7 +683,7 @@
   - Factory function: create_label_repository(db_path, use_mock) → Arc<dyn RelayLabelRepository>
   - If use_mock: return MockLabelRepository
   - Else: return SQLiteLabelRepository connected to db_path
-  - **File**: src/infrastructure/persistence/factory.rs
+  - **File**: `src/infrastructure/persistence/factory.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
   **Pseudocode**:
@@ -711,7 +711,7 @@
   - Determine test mode: cfg!(test) || env::var("CI").is_ok()
   - Call create_relay_controller() and create_label_repository()
   - Pass dependencies to RelayApi::new()
-  - **File**: src/startup.rs
+  - **File**: `src/startup.rs`
   - **Complexity**: Medium | **Uncertainty**: Low
 
   **Pseudocode**:
@@ -752,7 +752,7 @@
 - [ ] **T039d** [US1] [TDD] Register RelayApi in route aggregator
   - Add RelayApi to OpenAPI service
   - Tag: "Relays"
-  - **File**: src/startup.rs
+  - **File**: `src/startup.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
   **TDD Checklist**:
@@ -764,39 +764,39 @@
 - [ ] **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
+  - **File**: `tests/contract/test_relay_api.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [ ] **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
+  - **File**: `src/presentation/api/relay_api.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [ ] **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
+  - **File**: `tests/contract/test_relay_api.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [ ] **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
+  - **File**: `src/presentation/api/relay_api.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 ### Frontend Implementation
 
 - [ ] **T052** [P] [US1] [TDD] Create RelayDto TypeScript interface
   - Generate from OpenAPI spec or manually define
-  - **File**: frontend/src/types/relay.ts
+  - **File**: `frontend/src/types/relay.ts`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [ ] **T053** [P] [US1] [TDD] Create API client service
   - getAllRelays(): Promise<RelayDto[]>
   - toggleRelay(id: number): Promise<RelayDto>
-  - **File**: frontend/src/api/relayApi.ts
+  - **File**: `frontend/src/api/relayApi.ts`
   - **Complexity**: Low | **Uncertainty**: Low
 
 ---
@@ -810,7 +810,7 @@
 - [ ] **T046a** [US1] [TDD] Create useRelayPolling composable structure
   - Setup reactive refs: relays, isLoading, error, lastFetchTime
   - Define interval variable and fetch function signature
-  - **File**: frontend/src/composables/useRelayPolling.ts
+  - **File**: `frontend/src/composables/useRelayPolling.ts`
   - **Complexity**: Low | **Uncertainty**: Low
 
   **Pseudocode**:
@@ -850,7 +850,7 @@
   - Fetch relays and health status in parallel using Promise.all
   - Update reactive state on success
   - Handle errors gracefully, set isConnected based on success
-  - **File**: frontend/src/composables/useRelayPolling.ts
+  - **File**: `frontend/src/composables/useRelayPolling.ts`
   - **Complexity**: Medium | **Uncertainty**: Low
 
   **Pseudocode**:
@@ -886,7 +886,7 @@
   - startPolling(): Fetch immediately, then setInterval
   - stopPolling(): clearInterval and cleanup
   - Use onMounted/onUnmounted for automatic lifecycle management
-  - **File**: frontend/src/composables/useRelayPolling.ts
+  - **File**: `frontend/src/composables/useRelayPolling.ts`
   - **Complexity**: Medium | **Uncertainty**: Low
 
   **Pseudocode**:
@@ -926,7 +926,7 @@
 - [ ] **T046d** [US1] [TDD] Add connection status tracking
   - Track isConnected based on fetch success/failure
   - Display connection status in UI
-  - **File**: frontend/src/composables/useRelayPolling.ts
+  - **File**: `frontend/src/composables/useRelayPolling.ts`
   - **Complexity**: Low | **Uncertainty**: Low
 
   **Pseudocode**:
@@ -954,7 +954,7 @@
   - Props: relay (RelayDto)
   - Display relay ID, state, label
   - Emit toggle event on button click
-  - **File**: frontend/src/components/RelayCard.vue
+  - **File**: `frontend/src/components/RelayCard.vue`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [ ] **T056** [US1] [TDD] Create RelayGrid component
@@ -962,13 +962,13 @@
   - Render 8 RelayCard components
   - Handle toggle events by calling API
   - Display loading/error states
-  - **File**: frontend/src/components/RelayGrid.vue
+  - **File**: `frontend/src/components/RelayGrid.vue`
   - **Complexity**: Medium | **Uncertainty**: Low
 
 - [ ] **T057** [US1] [TDD] Integration test for US1
   - End-to-end test: Load page → see 8 relays → toggle relay 1 → verify state change
   - Use Playwright or Cypress
-  - **File**: frontend/tests/e2e/relay-control.spec.ts
+  - **File**: `frontend/tests/e2e/relay-control.spec.ts`
   - **Complexity**: Medium | **Uncertainty**: Medium
 
 **Checkpoint**: US1 MVP complete - users can view and toggle individual relays
@@ -984,49 +984,49 @@
 - [ ] **T058** [US2] [TDD] Write tests for BulkControlUseCase
   - Test: execute(BulkOperation::AllOn) turns all relays on
   - Test: execute(BulkOperation::AllOff) turns all relays off
-  - **File**: src/application/use_cases/bulk_control.rs
+  - **File**: `src/application/use_cases/bulk_control.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [ ] **T059** [US2] [TDD] Implement BulkControlUseCase
   - Call controller.write_all(state)
-  - **File**: src/application/use_cases/bulk_control.rs
+  - **File**: `src/application/use_cases/bulk_control.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [ ] **T060** [US2] [TDD] Define BulkOperation enum
   - Variants: AllOn, AllOff
-  - **File**: src/domain/relay.rs
+  - **File**: `src/domain/relay.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [ ] **T061** [US2] [TDD] Write contract tests for POST /api/relays/all/on
   - Test: Returns 200, all relays turn on
-  - **File**: tests/contract/test_relay_api.rs
+  - **File**: `tests/contract/test_relay_api.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [ ] **T062** [US2] [TDD] Implement POST /api/relays/all/on endpoint
   - Call BulkControlUseCase with AllOn
-  - **File**: src/presentation/api/relay_api.rs
+  - **File**: `src/presentation/api/relay_api.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [ ] **T063** [P] [US2] [TDD] Write contract tests for POST /api/relays/all/off
   - Test: Returns 200, all relays turn off
-  - **File**: tests/contract/test_relay_api.rs
+  - **File**: `tests/contract/test_relay_api.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [ ] **T064** [P] [US2] [TDD] Implement POST /api/relays/all/off endpoint
   - Call BulkControlUseCase with AllOff
-  - **File**: src/presentation/api/relay_api.rs
+  - **File**: `src/presentation/api/relay_api.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [ ] **T065** [US2] [TDD] Add bulk control buttons to frontend
   - Add "All On" and "All Off" buttons to RelayGrid component
   - Call API endpoints and refresh relay states
-  - **File**: frontend/src/components/RelayGrid.vue
+  - **File**: `frontend/src/components/RelayGrid.vue`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [ ] **T066** [US2] [TDD] Integration test for US2
   - Click "All On" → verify all 8 relays turn on
   - Click "All Off" → verify all 8 relays turn off
-  - **File**: frontend/tests/e2e/bulk-control.spec.ts
+  - **File**: `frontend/tests/e2e/bulk-control.spec.ts`
   - **Complexity**: Low | **Uncertainty**: Low
 
 **Checkpoint**: US2 complete - bulk controls functional
@@ -1043,46 +1043,46 @@
   - Test: Returns Healthy when controller is responsive
   - Test: Returns Degraded after 3 consecutive errors
   - Test: Returns Unhealthy after 10 consecutive errors
-  - **File**: src/application/use_cases/get_health.rs
+  - **File**: `src/application/use_cases/get_health.rs`
   - **Complexity**: Medium | **Uncertainty**: Low
 
 - [ ] **T068** [US3] [TDD] Implement GetHealthUseCase
   - Use HealthMonitor to track controller status
   - Return current HealthStatus
-  - **File**: src/application/use_cases/get_health.rs
+  - **File**: `src/application/use_cases/get_health.rs`
   - **Complexity**: Medium | **Uncertainty**: Low
 
 - [ ] **T069** [US3] [TDD] Define HealthDto
   - Fields: status ("healthy"/"degraded"/"unhealthy"), consecutive_errors (optional), reason (optional)
-  - **File**: src/presentation/dto/health_dto.rs
+  - **File**: `src/presentation/dto/health_dto.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [ ] **T070** [US3] [TDD] Write contract tests for GET /api/health
   - Test: Returns 200 with HealthDto
-  - **File**: tests/contract/test_health_api.rs
+  - **File**: `tests/contract/test_health_api.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [ ] **T071** [US3] [TDD] Implement GET /api/health endpoint
   - Call GetHealthUseCase, map to HealthDto
-  - **File**: src/presentation/api/health_api.rs
+  - **File**: `src/presentation/api/health_api.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [ ] **T072** [P] [US3] [TDD] Add firmware version display (optional)
   - If controller supports firmware_version(), display in UI
-  - **File**: frontend/src/components/DeviceInfo.vue
+  - **File**: `frontend/src/components/DeviceInfo.vue`
   - **Complexity**: Low | **Uncertainty**: Medium
   - **Note**: Device may not support this feature
 
 - [ ] **T073** [US3] [TDD] Create HealthIndicator component
   - Display connection status with color-coded indicator
   - Show firmware version if available
-  - **File**: frontend/src/components/HealthIndicator.vue
+  - **File**: `frontend/src/components/HealthIndicator.vue`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [ ] **T074** [US3] [TDD] Integrate HealthIndicator in RelayGrid
   - Fetch health status in useRelayPolling composable
   - Pass to HealthIndicator component
-  - **File**: frontend/src/components/RelayGrid.vue
+  - **File**: `frontend/src/components/RelayGrid.vue`
   - **Complexity**: Low | **Uncertainty**: Low
 
 **Checkpoint**: US3 complete - health monitoring visible
@@ -1099,35 +1099,35 @@
   - Test: execute(RelayId(1), "Pump") sets label
   - Test: execute with empty label returns error
   - Test: execute with 51-char label returns error
-  - **File**: src/application/use_cases/set_label.rs
+  - **File**: `src/application/use_cases/set_label.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [ ] **T076** [US4] [TDD] Implement SetLabelUseCase
   - Validate label with RelayLabel::new()
   - Call label_repository.set_label()
-  - **File**: src/application/use_cases/set_label.rs
+  - **File**: `src/application/use_cases/set_label.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [ ] **T077** [US4] [TDD] Write contract tests for PUT /api/relays/{id}/label
   - Test: Returns 200, label is persisted
   - Test: Returns 400 for invalid label
-  - **File**: tests/contract/test_relay_api.rs
+  - **File**: `tests/contract/test_relay_api.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [ ] **T078** [US4] [TDD] Implement PUT /api/relays/{id}/label endpoint
   - Parse id and label, call SetLabelUseCase
-  - **File**: src/presentation/api/relay_api.rs
+  - **File**: `src/presentation/api/relay_api.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [ ] **T079** [US4] [TDD] Add label editing to RelayCard component
   - Click label → show input field
   - Submit → call PUT /api/relays/{id}/label
-  - **File**: frontend/src/components/RelayCard.vue
+  - **File**: `frontend/src/components/RelayCard.vue`
   - **Complexity**: Medium | **Uncertainty**: Low
 
 - [ ] **T080** [US4] [TDD] Integration test for US4
   - Set label for relay 1 → refresh → verify label persists
-  - **File**: frontend/tests/e2e/relay-labeling.spec.ts
+  - **File**: `frontend/tests/e2e/relay-labeling.spec.ts`
   - **Complexity**: Low | **Uncertainty**: Low
 
 **Checkpoint**: US4 complete - relay labeling functional
@@ -1149,7 +1149,7 @@
   - Document request/response schemas
   - Add example values
   - Tag endpoints appropriately
-  - **File**: src/presentation/api/*.rs
+  - **File**: `src/presentation/api/*.rs`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [ ] **T083** [P] Run cargo clippy and fix all warnings
@@ -1172,19 +1172,19 @@
   - Document environment variables
   - Document Modbus device configuration
   - Add quickstart guide
-  - **File**: README.md
+  - **File**: `README.md`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [ ] **T088** [P] Create Docker image for backend
   - Multi-stage build with Rust
   - Include SQLite database setup
-  - **File**: Dockerfile
+  - **File**: `Dockerfile`
   - **Complexity**: Medium | **Uncertainty**: Low
 
 - [ ] **T089** [P] Create production settings/production.yaml
   - Configure for actual device IP
   - Set appropriate timeouts and retry settings
-  - **File**: settings/production.yaml
+  - **File**: `settings/production.yaml`
   - **Complexity**: Low | **Uncertainty**: Low
 
 - [ ] **T090** Deploy to production environment