feat(infrastructure): implement SQLite repository for relay labels

Add complete SQLite implementation of RelayLabelRepository trait with
all CRUD operations (get_label, save_label, delete_label, get_all_labels).

Key changes:
- Create infrastructure entities module with RelayLabelRecord struct
- Implement TryFrom traits for converting between database records and domain types
- Add From<sqlx::Error> and From<RelayLabelError> for RepositoryError
- Write comprehensive functional tests covering all repository operations
- Verify proper handling of edge cases (empty results, overwrites, max length)

TDD phase: GREEN - All repository trait tests now passing with SQLite implementation

Ref: T036 (specs/001-modbus-relay-control/tasks.md)

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

@@ -1,7 +1,7 @@
 mod label;
 pub use label::RelayLabelRepository;
 
-use super::types::RelayId;
+use super::types::{RelayId, RelayLabelError};
 
 /// Errors that can occur during repository operations.
 ///
@@ -16,3 +16,15 @@ pub enum RepositoryError {
     #[error("Relay not found: {0}")]
     NotFound(RelayId),
 }
+
+impl From<sqlx::Error> for RepositoryError {
+    fn from(value: sqlx::Error) -> Self {
+        Self::DatabaseError(value.to_string())
+    }
+}
+
+impl From<RelayLabelError> for RepositoryError {
+    fn from(value: RelayLabelError) -> Self {
+        Self::DatabaseError(value.to_string())
+    }
+}