Add support for maintenance mode

It would be great to add support for maintenance mode in ALBS:
* Based on a configuration flag, make all API calls to return a [503 - Service Unavailable](https://www.rfc-editor.org/rfc/rfc9110.html#name-503-service-unavailable), indicating the reason and a time before the next retry.
* Make ALBS frontend to show a generic "Maintenance Mode" page, indicating the information provided by the API (mostly, the reason why it's off).

We could make use of this [FastAPI Maintenance Mode Middleware](https://pypi.org/project/fastapi-maintenance-mode/) in backend. Not sure if there's anything for Vue.js though, needs some research.