Remove hardcoded huggingface provider from all podcast handlers

- script.py: set preferred_provider=None to respect GPT_PROVIDER
- research.py: set preferred_provider=None to respect GPT_PROVIDER
- Now all podcast handlers use GPT_PROVIDER env var

backend/api/podcast/handlers/research.py

@@ -148,7 +148,7 @@ Requirements:
                 prompt=prompt,
                 user_id=user_id,
                 json_struct=None,
-                preferred_provider="huggingface",
+                preferred_provider=None,
                 flow_type="premium_tool",
             )
             

backend/api/podcast/handlers/script.py

@@ -130,7 +130,7 @@ Guidelines:
             prompt=prompt,
             user_id=user_id,
             json_struct=None,
-            preferred_provider="huggingface",
+            preferred_provider=None,
             flow_type="premium_tool",
         )
     except HTTPException: