User initiated update process should not leave the page in maintenance mode when update gets interrupted by the user

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Is your feature request related to a problem? Please describe.
If an app gets updated (i.e. via file system) the next visitor (logged in or not) gets an update notification. It this user clicks on "Start Update" and closes the tab/window during this update process, the site is left in maintenance mode.

I think the main issue is, that this can be caused by externel users.

This happened here while testing, But I think similair situations can happen. I did not try it with a new Nc release, so I can't say if this happens also by interrupting the update process through the web updater after a new main release.

Describe the solution you'd like
Different approches seem to be supposable:

• Updates should not be able to get interrupted by closing the page
• App updates could be done by a background job (has challanges for sure)
• External users are not allowed to intiate the update process
• Only admins should be able to initiate updates
• updates via the frontend should be avoided
• IMHO at least an update from a not loggin user should be prevented in any case
• ...

Additional context

Text	Screenshot
First update an app (or change the version information in appinfo/info.xml
Visit login page
After click on "Start update"
While updating close the page
After that the site is left in maintenance mode and must be released via occ or changing the config file

Start by reproducing the interrupted update through the web updater: use appinfo/info.xml to trigger the update, select “Start update,” then close the page and inspect the resulting maintenance state. Review the update flow and the occ or config-file recovery path; done means an interrupted user-initiated update no longer leaves the site stuck in maintenance mode.