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/route/health.rs

@@ -30,7 +30,7 @@ impl HealthApi {
 
 #[tokio::test]
 async fn health_check_works() {
-    let app = crate::get_test_app();
+    let app = crate::get_test_app().await;
     let cli = poem::test::TestClient::new(app);
     let resp = cli.get("/api/health").send().await;
     resp.assert_status_is_ok();

backend/src/route/meta.rs

@@ -59,7 +59,7 @@ impl MetaApi {
 mod tests {
     #[tokio::test]
     async fn meta_endpoint_returns_correct_data() {
-        let app = crate::get_test_app();
+        let app = crate::get_test_app().await;
         let cli = poem::test::TestClient::new(app);
         let resp = cli.get("/api/meta").send().await;
         resp.assert_status_is_ok();
@@ -78,7 +78,7 @@ mod tests {
 
     #[tokio::test]
     async fn meta_endpoint_returns_200_status() {
-        let app = crate::get_test_app();
+        let app = crate::get_test_app().await;
         let cli = poem::test::TestClient::new(app);
         let resp = cli.get("/api/meta").send().await;
         resp.assert_status_is_ok();

backend/src/route/mod.rs

@@ -12,9 +12,10 @@ mod meta;
 use crate::settings::Settings;
 
 #[derive(Tags)]
-enum ApiCategory {
+pub enum ApiCategory {
     Health,
     Meta,
+    Relays,
 }
 
 pub(crate) struct Api {