fix: add return type annotation to MetaClassifierService constructor

app/services/meta_classifier.py

@@ -70,7 +70,7 @@ class MetaClassifierService:
     not available (during development before first training).
     """
 
-    def __init__(self):
+    def __init__(self) -> None:
         self._model = None
         self._scaler = None
         self._feature_cols: list[str] = []