* fix: purge deleted-stack notifications from panel and ticker
Stack delete already cascaded scans and drift but left notification_history rows, so the bell and Activity ticker kept showing the deleted stack. Purge those rows in the shared deletion lifecycle, invalidate connected clients, and drop node-scoped in-memory rows immediately.
* fix: target remote notification purge by hub node id
Remote stack-deleted invalidations always reconcile with hub rn.id, and
notification refetch preserves failed node slices instead of wiping them.
Anso