ML-009: add explainable predictions

Closes #20

app/ml/__init__.py

@@ -4,6 +4,7 @@ __all__ = [
     "FeatureStore",
     "FeatureVector",
     "FeatureModel",
+    "FeatureExplanation",
     "PredictionResult",
     "Predictor",
     "RetrainingResult",
@@ -13,6 +14,7 @@ __all__ = [
     "retrain_model",
 ]
 from app.ml.feature_store import FeatureStore, FeatureVector
+from app.ml.explanation import FeatureExplanation
 from app.ml.predictor import PredictionResult, Predictor
 from app.ml.retraining import RetrainingResult, RetrainingService, retrain_model
 from app.ml.training import FeatureModel, TrainedArtifact, TrainingPipeline