ML-007: add retraining pipeline and API

Closes #13

tests/ml/test_retraining.py

@@ -0,0 +1,39 @@
+from __future__ import annotations
+
+from pathlib import Path
+
+import pytest
+
+from app.ml.feature_store import FeatureVector
+from app.ml.registry.model_registry import ModelRegistry
+from app.ml.retraining import RetrainingService, retrain_model
+
+
+def _vector(sensor_id: str) -> FeatureVector:
+    return FeatureVector(sensor_id=sensor_id, values={"temperature": 21.0})
+
+
+def test_retraining_registers_new_artifact(tmp_path: Path) -> None:
+    registry = ModelRegistry(tmp_path)
+
+    result = retrain_model(registry, "home-model", [_vector("sensor.kitchen")])
+
+    assert result.replaced is False
+    assert registry.load_artifact("home-model") == result.artifact
+
+
+def test_retraining_replaces_existing_artifact(tmp_path: Path) -> None:
+    registry = ModelRegistry(tmp_path)
+    service = RetrainingService(registry)
+    service.retrain("home-model", [_vector("sensor.kitchen")])
+
+    result = service.retrain("home-model", [_vector("sensor.bedroom")])
+
+    assert result.replaced is True
+    assert result.artifact.supported_sensors == ("sensor.bedroom",)
+    assert ModelRegistry(tmp_path).load_artifact("home-model") == result.artifact
+
+
+def test_retraining_rejects_empty_training_data(tmp_path: Path) -> None:
+    with pytest.raises(ValueError, match="keine Trainingsdaten"):
+        retrain_model(ModelRegistry(tmp_path), "home-model", [])