Whitelist newly registered users

When using OIDC with a public IdP, the administrator usually doesn't want to enable auto registration, as not everyone should be able to have an account.

With disabled auto registration, an OIDC login is rejected and there is no way to whitelist one specific user. Gotify could create the account but keep it locked until an admin approves it, similar to Gitea/Forgejo. This could work as a new auto-register option (like false / true / locked) for both OIDC and normal sign-ups.

https://github.com/gotify/server/pull/977#discussion_r3409612051