Allow account recovery without access to an admin account

It's possible that the only admin loses their password. Gotify should provide a way to recover the admin account, or create a new admin account.

Options
- CLI commands for changing the password, creating a new user, changing admin status on user
- Consume a username:passwordhash file, and then update the username:passwordhash pairs

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I think we can ignore the possibility of losing the last admin due to OIDC login demoting a user. If permissions are handled by the IdP, then the admin should be able to configure the IdP to give a user admin permissions on next login.

No files, tests, or entry points are named. Start by comparing the proposed CLI and username:passwordhash approaches, then inspect the existing account, password, and admin-permission flows to determine the recovery boundary. Done means one agreed, secure way to recover or recreate the last admin without an existing admin session.

Written by the indexing model from the issue text.