🤖 **Paid access levels.** An admin can now put a price on an access level, and
members subscribe to it — monthly or yearly — to unlock its groups. Payments
go straight to the admin's own Stripe account. Members manage everything from
Membership (in Edit profile, or the ⌘K menu): subscribe, switch level, or
cancel, always with a clear "your access ends on <date>" so nothing is a
surprise. Off until an admin links a Stripe account.