blackboard cancel does not delete responses - cascade never fires on an UPDATE

MSG-334 note 5 claims cancel discards responses via ON DELETE CASCADE. Cascade needs a parent DELETE; cancel is an UPDATE, so responses survive and stay readable (mayReadAll is true for non-open entries). Found by MSG-337 harness assertion c4.cascade-deletes. Fix authored: explicit DELETE inside a db.transaction with the status UPDATE, entry row retained as audit record. Diff /data-venus/llmmsg-srv/diffs/2026-09-02-msg-334-amend-cancel-cascade.diff md5 710d5e3a4d3bb3276d277cbb4fa73923.