feat(domain): add ModbusAddress type and HealthStatus enum

Implements T025-T027 from TDD workflow (red-green-refactor):
- T025 (red): Tests for ModbusAddress with From<RelayId> conversion
- T026 (green): ModbusAddress newtype (#[repr(transparent)]) with offset mapping
- T027 (red+green): HealthStatus enum with state transitions

ModbusAddress wraps u16 and converts user-facing relay IDs (1-8) to
Modbus addresses (0-7) at the domain boundary. HealthStatus tracks
relay health with Healthy, Degraded, and Unhealthy states supporting
error tracking and recovery monitoring.

Ref: T025, T026, T027 (specs/001-modbus-relay-control)

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

@@ -1,3 +1,5 @@
+use std::fmt::Display;
+
 use thiserror::Error;
 
 /// Human-readable label for a relay.
@@ -45,3 +47,60 @@ impl Default for RelayLabel {
         Self(String::from("Unlabeled"))
     }
 }
+
+impl std::fmt::Display for RelayLabel {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{}", self.0)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_relay_label_new_valid_string() {
+        // Test: RelayLabel::new("Pump") → Ok
+        let result = RelayLabel::new("Pump".to_string());
+        assert!(result.is_ok());
+        assert_eq!(result.unwrap().as_str(), "Pump");
+    }
+
+    #[test]
+    fn test_relay_label_new_valid_max_length() {
+        // Test: RelayLabel::new("A".repeat(50)) → Ok
+        let label_str = "A".repeat(50);
+        let result = RelayLabel::new(label_str.clone());
+        assert!(result.is_ok());
+        assert_eq!(result.unwrap().as_str(), &label_str);
+    }
+
+    #[test]
+    fn test_relay_label_new_empty_fails() {
+        // Test: RelayLabel::new("") → Err(EmptyLabel)
+        let result = RelayLabel::new(String::new());
+        assert!(result.is_err());
+        assert!(matches!(result.unwrap_err(), RelayLabelError::Empty));
+    }
+
+    #[test]
+    fn test_relay_label_new_too_long_fails() {
+        // Test: RelayLabel::new("A".repeat(51)) → Err(LabelTooLong)
+        let label_str = "A".repeat(51);
+        let result = RelayLabel::new(label_str);
+        assert!(result.is_err());
+        assert!(matches!(result.unwrap_err(), RelayLabelError::TooLong(_)));
+    }
+
+    #[test]
+    fn test_relay_label_default() {
+        let label = RelayLabel::default();
+        assert_eq!(label.as_str(), "Unlabeled");
+    }
+
+    #[test]
+    fn test_relay_label_display() {
+        let label = RelayLabel::new("Test Label".to_string()).unwrap();
+        assert_eq!(format!("{label}"), "Test Label");
+    }
+}