Fix missing os import in hf_sentiment_provider causing crash on startup

backend/services/providers/hf_sentiment_provider.py

@@ -11,6 +11,7 @@ Uses HuggingFace Inference API for model inference.
 API Documentation: https://huggingface.co/docs/api-inference/
 """
 
+import os
 from __future__ import annotations
 from typing import Any, Dict, List, Optional