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/infrastructure/persistence/label_repository_tests.rs

@@ -12,11 +12,13 @@
 
 #[cfg(test)]
 mod relay_label_repository_contract_tests {
-    use crate::{domain::relay::{
-        repository::RelayLabelRepository,
-        types::{RelayId, RelayLabel},
-    }, infrastructure::persistence::label_repository::MockRelayLabelRepository};
-
+    use crate::{
+        domain::relay::{
+            repository::RelayLabelRepository,
+            types::{RelayId, RelayLabel},
+        },
+        infrastructure::persistence::label_repository::MockRelayLabelRepository,
+    };
 
     #[tokio::test]
     pub async fn test_get_label_returns_none_for_non_existent_relay() {
@@ -75,7 +77,6 @@ mod relay_label_repository_contract_tests {
         );
     }
 
-
     #[tokio::test]
     pub async fn test_save_label_succeeds() {
         let repo = MockRelayLabelRepository::new();
@@ -179,7 +180,6 @@ mod relay_label_repository_contract_tests {
         assert_eq!(retrieved.unwrap().as_str(), "X", "Label should match");
     }
 
-
     #[tokio::test]
     pub async fn test_delete_label_succeeds_for_existing_label() {
         let repo = MockRelayLabelRepository::new();
@@ -265,7 +265,6 @@ mod relay_label_repository_contract_tests {
         );
     }
 
-
     #[tokio::test]
     pub async fn test_get_all_labels_returns_empty_when_no_labels() {
         let repo = MockRelayLabelRepository::new();