feat(settings): add modbus and relay configuration structs

Add ModbusSettings with host, port, slave_id, and timeout_secs fields.
Add RelaySettings with label_max_length field.
Integrate both into Settings struct and load from settings/base.yaml
with test Modbus TCP configuration.

Ref: T003 (specs/001-modbus-relay-control)

settings/base.yaml

@@ -6,3 +6,12 @@ rate_limit:
   enabled: true
   burst_size: 10
   per_seconds: 60
+
+modbus:
+  host: "192.168.0.200"
+  port: 502
+  slave_id: 0
+  timeout_secs: 5
+
+relay:
+  label_max_length: 8

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

@@ -22,16 +22,16 @@
   - **Test**: Module declarations compile without errors
   - **Complexity**: Low | **Uncertainty**: Low
 
-- [ ] **T003** [P] [Setup] [TDD] Update settings.rs with Modbus configuration
+- [x] **T003** [P] [Setup] [TDD] Update settings.rs with Modbus configuration
   - Add ModbusSettings struct with `host`, `port`, `slave_id`, `timeout_secs` fields
   - Add RelaySettings struct with `label_max_length` field
   - Update Settings struct to include modbus and relay fields
   - **Test**: Settings loads from settings/base.yaml with test Modbus config
   - **Complexity**: Low | **Uncertainty**: Low
 
-- [ ] **T004** [P] [Setup] [TDD] Create settings/base.yaml with Modbus defaults
-  - Add modbus section: host: "192.168.0.200", port: 502, slave_id: 0, timeout_secs: 5
-  - Add relay section: label_max_length: 8
+- [x] **T004** [P] [Setup] [TDD] Create settings/base.yaml with Modbus defaults
+  - Add modbus section: `host: "192.168.0.200"`, `port: 502`, `slave_id: 0`, `timeout_secs: 5`
+  - Add relay section: `label_max_length: 8`
   - **Test**: Settings::new() loads config without errors
   - **Complexity**: Low | **Uncertainty**: Low
 

src/settings.rs

@@ -4,7 +4,8 @@
 //! - YAML configuration files (base.yaml and environment-specific files)
 //! - Environment variables (prefixed with APP__)
 //!
-//! Settings include application details, email server configuration, and environment settings.
+//! Settings include application details, Modbus connection parameters, relay configuration,
+//! rate limiting, and environment settings.
 
 /// Application configuration settings.
 ///
@@ -20,6 +21,10 @@ pub struct Settings {
     /// Rate limiting configuration
     #[serde(default)]
     pub rate_limit: RateLimitSettings,
+    /// Modbus configuration
+    pub modbus: ModbusSettings,
+    /// Relay configuration
+    pub relay: RelaySettings,
 }
 
 impl Settings {
@@ -156,6 +161,50 @@ 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::*;

src/startup.rs

@@ -181,6 +181,8 @@ mod tests {
                 burst_size: 100,
                 per_seconds: 60,
             },
+            modbus: crate::settings::ModbusSettings::default(),
+            relay: crate::settings::RelaySettings::default(),
         }
     }