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)

src/infrastructure/persistence/mod.rs

@@ -0,0 +1,7 @@
+//! Persistence layer implementations.
+//!
+//! This module contains the concrete implementations of repository traits
+//! for data persistence, including SQLite-based storage for relay labels.
+
+/// `SQLite` repository implementation for relay labels.
+pub mod sqlite_repository;