feat(persistence): initialize SQLite database module

- Add domain types: RelayId newtype and RepositoryError enum
- Implement SqliteRelayLabelRepository with in-memory test support
- Create relay_labels migration with SQLx compile-time verification
- Add comprehensive integration test suite (266 lines)

Ref: T006 (specs/001-modbus-relay-control)

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

@@ -41,7 +41,7 @@
   - **Test**: Schema file syntax is valid SQL
   - **Complexity**: Low | **Uncertainty**: Low
 
-- [ ] **T006** [P] [Setup] [TDD] Initialize SQLite database module
+- [x] **T006** [P] [Setup] [TDD] Initialize SQLite database module
   - Create infrastructure/persistence/mod.rs
   - Create infrastructure/persistence/sqlite_repository.rs with SqliteRelayLabelRepository struct
   - Implement SqliteRelayLabelRepository::new(path) using SqlitePool