Schedule deletion of the authenticated user's account (7-day cooldown)

Requests deletion of the calling account at entire-core. This is a soft delete: the account enters a 7-day cooldown, its sessions and tokens are revoked, and a reaper hard-deletes it (cascading its mirrored repos) once the cooldown elapses. Logging back in during the window cancels the deletion. On success the browser session is invalidated and its cookies cleared. Requires a valid session cookie and CSRF token.