fix: Purge user after deleting records that reference the user

[fregataa]

Issue

GQL purge_user mutation does not work.

1. The mutation spawns an asynchronous task to delete user's vfolders and immediately calls DELETE FROM users ... query, which violates a foreign key constraint set between vfolders.user and users.uuid.
2. The mutation checks whether the user owns any running kernels, but it does not check "keypairs" the user owns have running kernels.

How it changes

1. Delay DELETE FROM users ... query if the user owns vfolders where actual content remains. The mutation spawns an asynchronous task that calls DELETE FROM users ... query after finishing deletion of vfolders.
2. Check the user's keypairs own any running sessions.

Note

The purpose of this PR is a hotfix not a refactoring or enhancement.
The changed behavior does not ensure any deletion of vfolders and as it has been, it does not show any failure of vfolder deletion.

Checklist: (if applicable)

• Milestone metadata specifying the target backport version
• API server-client counterparts (e.g., manager API -> client SDK)

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[fregataa]

Close this PR since #2404 will cover the same issue