- Add background job that checks model availability every 5 hours
- Automatically removes unavailable models from fallback chain
- Adds unavailable models section in admin UI with blur styling
- Allows admins to re-add models when they become available again
- Extends model schema with available, lastChecked, unavailableSince fields
- Adds API endpoints: GET /api/admin/models/availability, POST /api/admin/models/:id/readd