fix: nested tag deletion

[nekomaido]

Summary

Fix nested tag deletion leaving orphaned tags hidden from the sidebar.

Deleting a parent tag now deletes all of its descendants. Existing tags whose
parent was deleted are restored to the root during migration, so they become
visible in the sidebar again.

Also prevents moving a tag under one of its own descendants, which would create
a circular hierarchy.

How to reproduce on the old version

1. Start with a fresh database.
2. Delete the Faved tag.
3. Refresh the page.

Old behavior

• Faved and its direct child Docs are deleted.
• Getting started and Guides remain, but their parent (Docs) no longer
  exists.
• Those tags no longer appear in the sidebar, so they cannot be edited or
  deleted there.
• They may still appear in tag-selection dropdowns.

New behavior

• Deleting Faved also deletes all of its descendant tags.
• Previously orphaned tags are moved to the root and shown in the sidebar.