User deletion fix

rhodecode/model/user.py

@@ -601,6 +601,10 @@ class UserModel(BaseModel):
                 if detach_artifacts:
                     self._detach_user_artifacts(user.username, user.artifacts, new_owner)
 
+                # Flush all detach operations to database before deleting user
+                # Ensures foreign keys are properly updated and prevents NULL constraint violations
+                self.sa.flush()
+
             user_data = user.get_dict()  # fetch user data before expire
 
             # we might change the user data with detach/delete, make sure