Add mutation that allows server shutdown

Is your feature request related to a problem?

To save electricity, I don't keep my Unraid server running 24/7. Instead, I can turn it on via Wake-on-LAN, and shut it down via the Unraid UI. I'd like to automate these tasks as part of a smart system, and that works fine for the WOL call, but I currently can't shutdown the server from an automation.

Describe the solution you'd like

A new Unraid API mutation to shutdown the server.

Describe alternatives you've considered

Installing an additional API on my server that supports this. I'm not aware of any existing solutions that allow this, and I would prefer first-party support.

Additional context

Environment (if relevant)

Unraid OS Version: 7.3.1

Pre-submission Checklist

• I have searched existing issues to ensure this feature hasn't already been requested
• This is not an Unraid Connect related feature (if it is, please submit via the support form instead)
• I have provided clear examples or use cases for the feature

The issue does not name files or tests. Start by locating the existing Unraid API mutation entry points and the server shutdown behavior used by the Unraid UI; the work is done when a first-party mutation can safely request server shutdown for the described automation use case and its behavior is covered by appropriate tests.