Enhance authentication middleware by adding HTTPException import

- Include HTTPException import in backend/middleware/auth.py to handle authentication errors more effectively.

backend/middleware/auth.py

@@ -8,7 +8,7 @@ Local development bypasses authentication entirely.
 import os
 import logging
 from typing import Optional, Dict, Any
-from fastapi import Request, Response
+from fastapi import Request, Response, HTTPException
 from starlette.middleware.base import BaseHTTPMiddleware
 from starlette.responses import RedirectResponse, JSONResponse
 from utils.environment import should_enable_auth, get_oauth_config, is_huggingface_space