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/tests/contract/test_relay_api.rs

@@ -0,0 +1,271 @@
+//! Contract tests for the Relay API HTTP endpoints.
+//!
+//! - **T048**: `GET /api/relays` contract tests
+//! - **T050**: `POST /api/relays/:id/toggle` contract tests
+
+use std::sync::Arc;
+
+use poem::{http::StatusCode, test::TestClient};
+use poem_openapi::OpenApiService;
+use sta::{
+    domain::relay::{
+        controller::RelayController,
+        repository::RelayLabelRepository,
+        types::{RelayId, RelayLabel, RelayState},
+    },
+    infrastructure::{
+        modbus::mock_controller::MockRelayController,
+        persistence::label_repository::MockRelayLabelRepository,
+    },
+    presentation::api::relay_api::RelayApi,
+};
+
+// -- Helpers --
+
+fn build_test_client(
+    controller: Arc<MockRelayController>,
+    repo: Arc<MockRelayLabelRepository>,
+) -> TestClient<impl poem::Endpoint> {
+    let relay_api = RelayApi::new(controller, repo);
+    let api_service = OpenApiService::new(relay_api, "STA", "0.1");
+    let app = poem::Route::new().nest("/api", api_service);
+    TestClient::new(app)
+}
+
+/// Creates a controller with all 8 relays initialised to `Off`.
+async fn all_relays_off() -> Arc<MockRelayController> {
+    let controller = Arc::new(MockRelayController::new());
+    for id in 1u8..=8 {
+        controller
+            .write_relay_state(RelayId::new(id).unwrap(), RelayState::Off)
+            .await
+            .unwrap();
+    }
+    controller
+}
+
+// ===========================================================================
+// T048: GET /api/relays
+// ===========================================================================
+
+/// T048 – Returns 200 OK.
+#[tokio::test]
+async fn get_all_relays_returns_200() {
+    let cli = build_test_client(all_relays_off().await, Arc::new(MockRelayLabelRepository::new()));
+
+    let resp = cli.get("/api/relays").send().await;
+
+    resp.assert_status_is_ok();
+}
+
+/// T048 – Returns an array of exactly 8 `RelayDto` objects.
+#[tokio::test]
+async fn get_all_relays_returns_array_of_8_relay_dtos() {
+    let cli = build_test_client(all_relays_off().await, Arc::new(MockRelayLabelRepository::new()));
+
+    let resp = cli.get("/api/relays").send().await;
+    resp.assert_status_is_ok();
+
+    let body: Vec<serde_json::Value> = resp.json().await.value().deserialize();
+    assert_eq!(body.len(), 8, "Expected 8 relays, got {}", body.len());
+}
+
+/// T048 – Relay IDs are 1 through 8, in ascending order.
+#[tokio::test]
+async fn get_all_relays_relay_ids_are_1_to_8_in_order() {
+    let cli = build_test_client(all_relays_off().await, Arc::new(MockRelayLabelRepository::new()));
+
+    let resp = cli.get("/api/relays").send().await;
+    let body: Vec<serde_json::Value> = resp.json().await.value().deserialize();
+
+    for (index, relay) in body.iter().enumerate() {
+        let expected_id = index + 1;
+        assert_eq!(
+            relay["id"], expected_id,
+            "Relay at index {index} should have id {expected_id}"
+        );
+    }
+}
+
+/// T048 – Every relay has a `state` field that is either `"on"` or `"off"`.
+#[tokio::test]
+async fn get_all_relays_each_relay_has_valid_state_field() {
+    let cli = build_test_client(all_relays_off().await, Arc::new(MockRelayLabelRepository::new()));
+
+    let resp = cli.get("/api/relays").send().await;
+    let body: Vec<serde_json::Value> = resp.json().await.value().deserialize();
+
+    for relay in &body {
+        let state = relay["state"].as_str().expect("state should be a string");
+        assert!(
+            state == "on" || state == "off",
+            "state must be 'on' or 'off', got '{state}'"
+        );
+    }
+}
+
+/// T048 – Every relay has a `label` field (string).
+#[tokio::test]
+async fn get_all_relays_each_relay_has_label_field() {
+    let cli = build_test_client(all_relays_off().await, Arc::new(MockRelayLabelRepository::new()));
+
+    let resp = cli.get("/api/relays").send().await;
+    let body: Vec<serde_json::Value> = resp.json().await.value().deserialize();
+
+    for relay in &body {
+        assert!(relay["label"].is_string(), "label should be a string field");
+    }
+}
+
+/// T048 – Relay states in the response match the controller's actual states.
+#[tokio::test]
+async fn get_all_relays_states_reflect_controller_state() {
+    let controller = all_relays_off().await;
+    controller
+        .write_relay_state(RelayId::new(1).unwrap(), RelayState::On)
+        .await
+        .unwrap();
+    controller
+        .write_relay_state(RelayId::new(3).unwrap(), RelayState::On)
+        .await
+        .unwrap();
+
+    let cli = build_test_client(controller, Arc::new(MockRelayLabelRepository::new()));
+    let resp = cli.get("/api/relays").send().await;
+    let body: Vec<serde_json::Value> = resp.json().await.value().deserialize();
+
+    assert_eq!(body[0]["state"], "on",  "Relay 1 should be on");
+    assert_eq!(body[1]["state"], "off", "Relay 2 should be off");
+    assert_eq!(body[2]["state"], "on",  "Relay 3 should be on");
+    assert_eq!(body[3]["state"], "off", "Relay 4 should be off");
+}
+
+/// T048 – A relay with a persisted label returns that label.
+#[tokio::test]
+async fn get_all_relays_relay_with_label_returns_label() {
+    let repo = Arc::new(MockRelayLabelRepository::new());
+    repo.save_label(
+        RelayId::new(2).unwrap(),
+        RelayLabel::new("Water Pump".to_string()).unwrap(),
+    )
+    .await
+    .unwrap();
+
+    let cli = build_test_client(all_relays_off().await, repo);
+    let resp = cli.get("/api/relays").send().await;
+    let body: Vec<serde_json::Value> = resp.json().await.value().deserialize();
+
+    assert_eq!(body[1]["label"], "Water Pump");
+}
+
+// ===========================================================================
+// T050: POST /api/relays/:id/toggle
+// ===========================================================================
+
+/// T050 – Returns 200 OK with a `RelayDto` body.
+#[tokio::test]
+async fn toggle_relay_returns_200_with_relay_dto() {
+    let cli = build_test_client(all_relays_off().await, Arc::new(MockRelayLabelRepository::new()));
+
+    let resp = cli.post("/api/relays/1/toggle").send().await;
+
+    resp.assert_status_is_ok();
+    let body: serde_json::Value = resp.json().await.value().deserialize();
+    assert!(body["id"].is_number());
+    assert!(body["state"].is_string());
+    assert!(body["label"].is_string());
+}
+
+/// T050 – Returns 404 for relay id 0 (below valid range).
+#[tokio::test]
+async fn toggle_relay_returns_404_for_id_below_range() {
+    let cli = build_test_client(all_relays_off().await, Arc::new(MockRelayLabelRepository::new()));
+
+    let resp = cli.post("/api/relays/0/toggle").send().await;
+
+    resp.assert_status(StatusCode::NOT_FOUND);
+}
+
+/// T050 – Returns 404 for relay id 9 (above valid range).
+#[tokio::test]
+async fn toggle_relay_returns_404_for_id_above_range() {
+    let cli = build_test_client(all_relays_off().await, Arc::new(MockRelayLabelRepository::new()));
+
+    let resp = cli.post("/api/relays/9/toggle").send().await;
+
+    resp.assert_status(StatusCode::NOT_FOUND);
+}
+
+/// T050 – State changes from `Off` to `On` and response reflects new state.
+#[tokio::test]
+async fn toggle_relay_off_to_on_response_shows_on() {
+    let cli = build_test_client(all_relays_off().await, Arc::new(MockRelayLabelRepository::new()));
+
+    let resp = cli.post("/api/relays/1/toggle").send().await;
+    resp.assert_status_is_ok();
+
+    let body: serde_json::Value = resp.json().await.value().deserialize();
+    assert_eq!(body["state"], "on");
+}
+
+/// T050 – State changes from `On` to `Off` and response reflects new state.
+#[tokio::test]
+async fn toggle_relay_on_to_off_response_shows_off() {
+    let controller = Arc::new(MockRelayController::new());
+    controller
+        .write_relay_state(RelayId::new(5).unwrap(), RelayState::On)
+        .await
+        .unwrap();
+
+    let cli = build_test_client(controller, Arc::new(MockRelayLabelRepository::new()));
+
+    let resp = cli.post("/api/relays/5/toggle").send().await;
+    resp.assert_status_is_ok();
+
+    let body: serde_json::Value = resp.json().await.value().deserialize();
+    assert_eq!(body["state"], "off");
+}
+
+/// T050 – State actually changes in the underlying controller, not just in the response.
+#[tokio::test]
+async fn toggle_relay_state_actually_changes_in_controller() {
+    let controller = all_relays_off().await;
+    let relay_id = RelayId::new(3).unwrap();
+
+    let cli = build_test_client(controller.clone(), Arc::new(MockRelayLabelRepository::new()));
+    cli.post("/api/relays/3/toggle").send().await;
+
+    let state = controller.read_relay_state(relay_id).await.unwrap();
+    assert_eq!(state, RelayState::On, "Relay 3 should be On in the controller after toggle");
+}
+
+/// T050 – Response includes the correct relay id.
+#[tokio::test]
+async fn toggle_relay_response_includes_correct_relay_id() {
+    let cli = build_test_client(all_relays_off().await, Arc::new(MockRelayLabelRepository::new()));
+
+    let resp = cli.post("/api/relays/4/toggle").send().await;
+    resp.assert_status_is_ok();
+
+    let body: serde_json::Value = resp.json().await.value().deserialize();
+    assert_eq!(body["id"], 4);
+}
+
+/// T050 – Response includes a persisted label.
+#[tokio::test]
+async fn toggle_relay_response_includes_label_when_set() {
+    let repo = Arc::new(MockRelayLabelRepository::new());
+    repo.save_label(
+        RelayId::new(6).unwrap(),
+        RelayLabel::new("Heater".to_string()).unwrap(),
+    )
+    .await
+    .unwrap();
+
+    let cli = build_test_client(all_relays_off().await, repo);
+    let resp = cli.post("/api/relays/6/toggle").send().await;
+    resp.assert_status_is_ok();
+
+    let body: serde_json::Value = resp.json().await.value().deserialize();
+    assert_eq!(body["label"], "Heater");
+}