Add OAuth provider logout support

### Describe the feature you'd like

When using an OAuth provider (In my case Pocket ID), the provider remains logged in when I log out of Karakeep. This means that if you immediately go back to login to Karakeep with the OAuth provider, it does so automatically as it is already logged in. I would like to be able to log out of my provider when logging out of Karakeep.

### Describe the benefits this would bring to existing Karakeep users

It creates better security for users and greater convenience. It enables them to log out of Karakeep and the OAuth provider at the same time.

### Can the goal of this request already be achieved via other means?

Not really. Users could manually log out of their OAuth Provider but that creates user friction and adds unnecessary steps.

### Have you searched for an existing open/closed issue?

- [x] I have searched for existing issues and none cover my fundamental request

### Additional context

_No response_

No files, tests, or entry points are named. Start by locating Karakeep's OAuth login and logout handling and determine whether the provider supports logout through its protocol. Done means logging out of Karakeep also logs the user out of the OAuth provider when supported, without breaking normal logout.

Written by the indexing model from the issue text.