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)

Cargo.toml

@@ -31,5 +31,8 @@ tokio-modbus = { version = "0.17.0", default-features = false, features = ["tcp"
 tracing = "0.1.44"
 tracing-subscriber = { version = "0.3.22", features = ["fmt", "std", "env-filter", "registry", "json", "tracing-log"] }
 
+[dev-dependencies]
+tempfile = "3.15.0"
+
 [lints.rust]
 unexpected_cfgs = { level = "warn", check-cfg = ['cfg(tarpaulin_include)'] }