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/dto/relay_dto.rs

@@ -126,10 +126,7 @@ mod tests {
         let dto = RelayDto::from(relay);
 
         let json = serde_json::to_string(&dto).unwrap();
-        assert_eq!(
-            json,
-            r#"{"id":7,"state":"on","label":"Test Relay"}"#
-        );
+        assert_eq!(json, r#"{"id":7,"state":"on","label":"Test Relay"}"#);
     }
 
     #[test]