Delete a topic cluster

Deletes a topic cluster. DESTRUCTIVE beyond the cluster itself: every PLANNED content item in it is deleted along with it and cannot be recovered, because content planner rows cascade on the topic. Its keyword and prompt links are removed too, though the keywords and prompts themselves stay in the team's lists. Published and generated articles are kept, but lose their cluster. Tell the user how many planned items will be lost and get explicit confirmation before calling this.