Feat: Reject default admin account log in to normal services (infiniflow#11994)

### What problem does this PR solve?

Feat: Reject default admin account log in to normal services
infiniflow#11854
infiniflow#11673

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

Author: Magicbook1108

api/apps/user_app.py

@@ -98,6 +98,9 @@ async def login():
         return get_json_result(data=False, code=RetCode.AUTHENTICATION_ERROR, message="Unauthorized!")
 
     email = json_body.get("email", "")
+    if email == "admin@ragflow.io":
+        return get_json_result(data=False, code=RetCode.AUTHENTICATION_ERROR, message="Default admin account cannot be used to login normal services!")
+    
     users = UserService.query(email=email)
     if not users:
         return get_json_result(