Add maintenance shutdown API #733
Description
Add an admin API to toggle maintenance mode: /admin/actions/maintenance
Accepts payload:
- status: MaintenanceStatus (None, Readonly, Shutdown)
When the status is Shutdown, all non-admin endpoints return 503 - Service Unavailable, with the maintenance status as payload. The main purpose is to avoid issues during data migrations/DNS updates.
When the status is Readonly, the serve_web_site fallback and site owner GET endpoints are still available.