How to use GitHub

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Is your feature request related to a problem? Please describe.
A clear and concise description of what the problem is. Ex.

• In a self hosted environment, it is usually for private use and exposed to the internet.
• In the current setup, Monica lets you create accounts freely through email verification. This means, anyone in the world can freely create an account without the knowledge of the admin, unless the admin looks through logs, checks the database, or see what the mail server sent the emails to.
• Although, turning off mailing service is possible, it would be a hassle to ask an admin to give you the verification link from their Monica logs when the user has time to create an account.

Describe the solution you'd like
A timed and or N amount of use invite link to let users have access to the register page to create their account.

Describe alternatives you've considered (optional)
Normal registration but upon verifying, needs admin approval to gain access. Though it could have unwanted random approvals from random people in the event the domain was accidentally publicly revealed and spread in the internet..

Additional context
Add any other context or screenshots about the feature request here. The more context, the better.
JFA-Go does something similar for Jellyfin, it creates an invite link for users to freely create their Jellyfin account.