Delete Niche

Delete all sessions for a given niche — A7: route EACH through _teardown_session (cancel + settle-via-release + delete FK children in order), not a raw sa_delete(Session). The raw delete 500'd on any post-CP3 session (outputs/story_picks/scheduled_posts FKs have no ON DELETE) and stranded held credits on pre-CP3 ones (credit_transactions.session_id SET NULL → the open reservation could never be reconstructed/released). Commit per session so one bad row doesn't roll back the batch.