ML-007: add retraining pipeline and API

Closes #13

tests/api/test_ml_routes.py

@@ -50,3 +50,60 @@ def test_unsupported_sensor_returns_422(tmp_path: Path) -> None:
         )
 
     assert response.status_code == 422
+
+
+def test_retrain_creates_and_replaces_persisted_model(tmp_path: Path) -> None:
+    from app.ml.registry.model_registry import ModelRegistry
+
+    registry = ModelRegistry(tmp_path)
+    with TestClient(app) as client:
+        app.state.registry = registry
+        created = client.post(
+            "/ml/retrain",
+            json={
+                "modelId": "home-model",
+                "samples": [
+                    {
+                        "sensor_id": "sensor.kitchen",
+                        "values": {"temperature": 21.0},
+                    }
+                ],
+            },
+        )
+        replaced = client.post(
+            "/ml/retrain",
+            json={
+                "modelId": "home-model",
+                "samples": [
+                    {
+                        "sensor_id": "sensor.bedroom",
+                        "values": {"temperature": 18.0},
+                    }
+                ],
+            },
+        )
+
+    assert created.status_code == 200
+    assert created.json() == {
+        "model_id": "home-model",
+        "supported_sensors": ["sensor.kitchen"],
+        "replaced": False,
+    }
+    assert replaced.status_code == 200
+    assert replaced.json() == {
+        "model_id": "home-model",
+        "supported_sensors": ["sensor.bedroom"],
+        "replaced": True,
+    }
+    restarted = ModelRegistry(tmp_path)
+    assert restarted.load_artifact("home-model").supported_sensors == ("sensor.bedroom",)
+
+
+def test_retrain_rejects_empty_samples() -> None:
+    with TestClient(app) as client:
+        response = client.post(
+            "/ml/retrain",
+            json={"modelId": "home-model", "samples": []},
+        )
+
+    assert response.status_code == 422