fix: add 'cancelled' to housekeeping_tasks status check constraint

DB constraint only allowed pending/in_progress/done/skipped, causing
500 error when trying to cancel a maintenance task.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/migrations/032_hk_status_cancelled.sql

@@ -0,0 +1,6 @@
+ALTER TABLE housekeeping_tasks
+  DROP CONSTRAINT IF EXISTS housekeeping_tasks_status_check;
+
+ALTER TABLE housekeeping_tasks
+  ADD CONSTRAINT housekeeping_tasks_status_check
+  CHECK (status IN ('pending', 'in_progress', 'done', 'skipped', 'cancelled'));