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/lib.rs

@@ -85,7 +85,7 @@ pub mod presentation;
 
 type MaybeListener = Option<poem::listener::TcpListener<String>>;
 
-fn prepare(listener: MaybeListener) -> startup::Application {
+async fn prepare(listener: MaybeListener) -> startup::Application {
     dotenvy::dotenv().ok();
     let settings = settings::Settings::new().expect("Failed to read settings");
     if !cfg!(test) {
@@ -98,7 +98,8 @@ fn prepare(listener: MaybeListener) -> startup::Application {
         "Using these settings: {:?}",
         settings
     );
-    let application = startup::Application::build(settings, listener);
+    let application = startup::Application::build(settings, listener).await
+        .expect("Failed to build application");
     tracing::event!(
         target: "backend",
         tracing::Level::INFO,
@@ -124,7 +125,7 @@ fn prepare(listener: MaybeListener) -> startup::Application {
 /// an I/O error during runtime (e.g., port already in use, network issues).
 #[cfg(not(tarpaulin_include))]
 pub async fn run(listener: MaybeListener) -> Result<(), std::io::Error> {
-    let application = prepare(listener);
+    let application = prepare(listener).await;
     application.make_app().run().await
 }
 
@@ -137,7 +138,7 @@ fn make_random_tcp_listener() -> poem::listener::TcpListener<String> {
 }
 
 #[cfg(test)]
-fn get_test_app() -> startup::App {
+async fn get_test_app() -> startup::App {
     let tcp_listener = make_random_tcp_listener();
-    prepare(Some(tcp_listener)).make_app().into()
+    prepare(Some(tcp_listener)).await.make_app().into()
 }