Improve tenant deletion safety and cascading

This page is a capture in the next bucket of the product backlog — a pre-sprint idea, not yet pulled into a sprint as a story.

Tenant deletion needs proper safeguards and cascading behaviour.

• Deleting any tenant should show an impact summary before proceeding.
• Deleting a tenant should trigger cascading deletion of associated parties.
• Deleting the root tenant should return an error (prevent accidental deletion).