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/src/presentation/mod.rs

@@ -100,6 +100,6 @@
 /// This module contains DTO structures that are used to serialize domain
 /// objects for API responses, providing a clean separation between internal
 /// domain models and external API contracts.
+pub mod api;
 pub mod dto;
-
 pub mod error;