Fix: Add /oauth-callback endpoint for OAuth flow

🐛 Problem:
- OAuth callback URL /oauth-callback returns 404 error
- HF redirects to /oauth-callback but backend only has /auth/callback
- Frontend and backend using different callback paths

✅ Solution:
- Added new /auth/oauth-callback endpoint route
- Unified callback handler handle_oauth_callback()
- Updated redirect_uri to use /oauth-callback consistently
- Keep legacy /auth/callback for compatibility

🔧 Technical Changes:
Backend:
-

@router
.get('/oauth-callback') -> oauth_callback_new()
- Refactored oauth_callback logic into shared handle_oauth_callback()
- Updated all redirect_uri to use /oauth-callback path
- Maintains backward compatibility with existing /auth/callback

🎯 Expected Result:
- OAuth callback should work correctly
- No more 404 errors after HF authorization
- Successful authentication and redirect to app

backend/routers/auth.py

@@ -114,7 +114,7 @@ async def login(request: Request):
     else:
         base_url = str(request.base_url).rstrip('/')
         
-    redirect_uri = f"{base_url}/auth/callback"
+    redirect_uri = f"{base_url}/oauth-callback"
     
     # Build authorization URL
     auth_url = (
@@ -131,6 +131,17 @@ async def login(request: Request):
 
 @router.get("/callback")
 async def oauth_callback(request: Request, code: str, state: str):
+    """Legacy callback endpoint (keep for compatibility)"""
+    return await handle_oauth_callback(request, code, state)
+
+
+@router.get("/oauth-callback") 
+async def oauth_callback_new(request: Request, code: str, state: str):
+    """New OAuth callback endpoint for direct OAuth flow"""
+    return await handle_oauth_callback(request, code, state)
+
+
+async def handle_oauth_callback(request: Request, code: str, state: str):
     """
     Handle OAuth callback from Hugging Face.
     """
@@ -178,7 +189,7 @@ async def oauth_callback(request: Request, code: str, state: str):
     else:
         base_url = str(request.base_url).rstrip('/')
         
-    redirect_uri = f"{base_url}/auth/callback"
+    redirect_uri = f"{base_url}/oauth-callback"
     
     try:
         token_response = requests.post(