feat: wire relay API with dependency injection

- split settings module into per-struct files
- add DatabaseSettings with default in-memory SQLite path
- implement RelayApi struct with GET /relays and POST
  /relays/{id}/toggle
- wire create_relay_controller and create_label_repository into
  Application::build() with mock/real selection via cfg!(test) || CI
- register RelayApi in OpenApiService alongside existing APIs

backend/Cargo.toml

@@ -37,5 +37,9 @@ tracing-subscriber = { version = "0.3.22", features = ["fmt", "std", "env-filter
 [dev-dependencies]
 tempfile = "3.15.0"
 
+[[test]]
+name = "relay_api_contract"
+path = "tests/contract/test_relay_api.rs"
+
 [lints.rust]
 unexpected_cfgs = { level = "warn", check-cfg = ['cfg(tarpaulin_include)'] }