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/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 {