Refuse to run the Updater if the upgrade cycle hasn't finished

In the web UI, when the "Go back to your Nextcloud instance to finish the update" comes up (which is supposed to be used to visit the instance and run the db migrations/app upgrades), it appears to be possible to merely hit reload and start the next update cycle.

This shouldn't be permitted.

Also, it's what turned up #566 in the wild (I think). Because the repair step (for the stray .step file we're leaving around) is added during the db migrations/etc.

This leaves the instance stuck in step 12 until manual intervention is taken.

This is currently possible because the Updater doesn't look at the database to check what version is currently in place, but merely looks at the version.php

Since we can't have the Updater connect to the db, we need a reliable mechanism for it to detect whether the upgrade cycle was fully completed.

Start by tracing the updater flow behind the web UI message about returning to the Nextcloud instance, then inspect how version.php, database migrations, and the stray .step file represent upgrade progress. Define a reliable completion check and verify that reloading cannot begin another cycle before migrations and app upgrades finish, including the step-12 failure described here.