Maintenance mode feature

[ferishili]

This PR fixes #963 and introduces a new maintenance mode feature with backend and frontend integration, along with a series of improvements for configuration handling and system behavior during different maintenance states.

Key Features

• Database Migration

  • Adds 3 new columns to the oc_config table (via migration no. 113).

• Admin Server Settings

  • A new section to control the maintenance mode:
    • off or auto: disabled or automatically sync with OC (WIP).
    • on: No interaction with Opencast allowed. The frontend hides all videos and functionalities.
    • read-only: Only playback via the presentation node is allowed; videos are visible but not editable.
    • fallback presentation node URL: Optional; if not set, it is derived from the search endpoint.
    • maintenance message: Optional message shown to users in the frontend.

• API Behavior

  • During maintenance mode, related API clients are not instantiated, enforcing no backend interaction and returning an internal 503 error when needed.
  • In read-only mode, only search and play endpoints are permitted.

• Frontend Enhancements

  • Refactored how simple config values are read in Vue:
    • Now fetched once in App.vue and reused across components.
    • Introduces a new composable: useSimpleConfigChecker for periodic updates and page reloads on critical config changes.

• Cronjob Handling

  • All cronjobs are disabled during maintenance mode.

• LTI Behavior

  • on: LTI access is fully disabled.
  • read-only: LTI access is allowed only via the presentation node.

...

[WIP] Next Steps

• Opencast is expected to expose an official maintenance mode endpoint (not finalized yet).
• Once available, a cronjob will be added to sync the maintenance mode state from Opencast and reflect the changes in this system.

[ferishili]

@tgloeggl, @dennis531 since it is a new feature with a lot of changes, your feedback is very important!

[github-actions]

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Please resolve those so we can review the pull request.
Thanks.