Now that subscriptions survive `done` (completion is reversible —
on every 5s notifier tick forever. Add
kanban_db.purge_stale_done_notify_subs(): one DELETE removing subs
whose task has been done with no new events past a retention window
(age = latest task event, falling back to completed_at/created_at, so
any activity exempts the task; a reopened task is exempt by status
alone). The notifier watcher runs it per board once at startup and at
most hourly, re-reading kanban.done_sub_retention_days (config.yaml,
default 30; 0 disables) at each sweep.
