ML-009: add explainable predictions

Closes #20

tests/api/test_ml_routes.py

@@ -143,6 +143,10 @@ def test_predict_returns_numeric_forecast_and_confidence(tmp_path: Path) -> None
     assert response.json()["predictions"] == {"temperature": 22.0}
     assert 0.0 < response.json()["confidence"] <= 1.0
     assert response.json()["model_type"] == "statistical_baseline"
+    explanation = response.json()["explanations"]["temperature"]
+    assert explanation["direction"] == "steigend"
+    assert explanation["change"] == 1.0
+    assert explanation["sample_count"] == 2
 
 
 def test_evaluate_returns_real_error_metrics(tmp_path: Path) -> None: