Admin Retire Mailboxes

Permanently take mailboxes out of the pool (terminal retired status).

For boxes we are done with (e.g. returning to the vendor). Unlike a pause, survives the provider sync and cancels queued replies too.

Staff-only on top of the router's require_org_admin: the router gate passes any customer who owns their own org, while this action is irreversible and fleet-wide (outreach_mailboxes is the shared operator pool, unscoped by org) — it must never be reachable from a self-serve account.