Add an authenticated POST /api/v1/invites endpoint that mints a
single-use, 7-day registration link after the first-user bootstrap.
Expose it via a new "Invite" section on the admin page and a
`brewlog invite create` CLI command.
The registration_tokens table, repository, and NewRegistrationToken
already supported additional tokens; this adds the authenticated
surfaces (session cookie or bearer token) to trigger creation.
