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/infrastructure/persistence/mod.rs

@@ -12,3 +12,5 @@ pub mod label_repository_tests;
 
 /// `SQLite` repository implementation for relay labels.
 pub mod sqlite_repository;
+
+pub mod entities;