feat(domain): implement RelayController trait and error handling

Add RelayController async trait (T030) defining interface for Modbus
relay operations with methods for read/write state, bulk operations,
connection checks, and firmware queries.

Implement ControllerError enum (T031) with variants for connection
failures, timeouts, Modbus exceptions, and invalid relay IDs.

Provide MockRelayController (T029) in-memory implementation using
Arc<Mutex<HashMap>> for thread-safe state storage with timeout
simulation for error handling tests.

Add RelayLabelRepository trait abstraction.

Ref: T029 T030 T031 (specs/001-modbus-relay-control/tasks.md)

backend/src/domain/relay/repository.rs

@@ -1,4 +1,6 @@
-use super::types::RelayId;
+use async_trait::async_trait;
+
+use super::types::{RelayId, RelayLabel};
 
 /// Errors that can occur during repository operations.
 ///
@@ -13,3 +15,39 @@ pub enum RepositoryError {
     #[error("Relay not found: {0}")]
     NotFound(RelayId),
 }
+
+/// Repository trait for persisting and retrieving relay labels.
+///
+/// This trait abstracts data persistence operations for relay labels,
+/// enabling different storage implementations (e.g., `SQLite`, `PostgreSQL`, in-memory).
+/// Implementations must be thread-safe (`Send + Sync`) for use in async contexts.
+#[async_trait]
+pub trait RelayLabelRepository: Send + Sync {
+    /// Retrieves the label for a specific relay.
+    ///
+    /// Returns `None` if no label has been set for the relay.
+    ///
+    /// # Errors
+    ///
+    /// Returns `RepositoryError::DatabaseError` if the database operation fails.
+    async fn get_label(&self, id: RelayId) -> Result<Option<RelayLabel>, RepositoryError>;
+
+    /// Saves or updates the label for a specific relay.
+    ///
+    /// If a label already exists for the relay, it will be overwritten.
+    ///
+    /// # Errors
+    ///
+    /// Returns `RepositoryError::DatabaseError` if the database operation fails.
+    async fn save_label(&self, id: RelayId, label: RelayLabel) -> Result<(), RepositoryError>;
+
+    /// Retrieves all relay labels from the repository.
+    ///
+    /// Returns a vector of tuples containing relay IDs and their corresponding labels.
+    /// Relays without labels are not included in the result.
+    ///
+    /// # Errors
+    ///
+    /// Returns `RepositoryError::DatabaseError` if the database operation fails.
+    async fn get_all_labels(&self) -> Result<Vec<(RelayId, RelayLabel)>, RepositoryError>;
+}