fix: add return type annotation to Wav2Vec2DetectorService constructor

app/services/wav2vec2_detector.py

@@ -47,7 +47,7 @@ class Wav2Vec2DetectorService:
     On CPU, inference takes ~0.5s for 30s audio.
     """
 
-    def __init__(self, model_path: Optional[Path] = None):
+    def __init__(self, model_path: Optional[Path] = None) -> None:
         self._model = None
         self._device = None
         self._initialized = False