[Suggestion] Add invitation-based registration system

[Wikijito7]

Is your feature request related to a problem? Please describe.
The web frontend plans to have an invitation-only registration flow (/invite/[id]), but there is no backend support for generating or validating registration invites.

Describe the solution you'd like
Implement the following endpoints:

• POST /admin/invite — Generate a new invite token (admin-only, authenticated)
• GET /invite/{token} — Validate an invite token and return its details
• POST /register/{token} — Register a new user using an invite token (creates user + consumes the invite)

Invite tokens should:

• Be UUID-based or similar
• Have an expiration date
• Be single-use (consumed after registration)
• Track who created the invite

Describe alternatives you've considered

• Public registration with email verification (not desired — invites only)
• Hardcoding invite codes in config (not scalable)

Additional context
Replaces the need for a public /register endpoint or email verification flow. The web frontend will have an /invite/[id] page that calls GET /invite/{token} to validate and POST /register/{token} to complete registration.