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/application/use_cases/get_all_relays.rs

@@ -265,9 +265,15 @@ mod tests {
         for (index, relay) in result.iter().enumerate() {
             let relay_num = index + 1;
             if relay_num % 2 == 1 {
-                assert!(relay.label().is_some(), "Relay {relay_num} should have label");
+                assert!(
+                    relay.label().is_some(),
+                    "Relay {relay_num} should have label"
+                );
             } else {
-                assert!(relay.label().is_none(), "Relay {relay_num} should not have label");
+                assert!(
+                    relay.label().is_none(),
+                    "Relay {relay_num} should not have label"
+                );
             }
         }
     }