Config option to disable signup form

For some instances we want the login to be SSO/Invite only.

We currently have the isWkorgInstance switch, which influences the signup forms, but this particular option is not currently available.

Additional context: https://scm.slack.com/archives/C5AKLAV0B/p1752050400786589

Let’s have a close look at the current logic again, I’m not 100% sure what the current implementation offers.