Log out if session no longer exists

There are a few ways that a user's session could be deleted or no longer exist before the session expires without the user logging out, none of them extremely likely. One example: restoring a backup will cause any sessions created after the backup was created to no longer exist.

Frontend automatically logs out before a session expires, but it doesn't account for the possibility that a session could stop existing before it expires. If that happens, then Frontend will continue sending requests, but it will show the resulting 401 errors. Instead, I think it'd be better for Frontend to log out immediately in that case. (Should it just log out whenever it receives a 401? Or maybe receiving a 401 should cause it to try to check the existence of the session?)