- extras/test_backup_kopia.sh: stop → restore → compare → roll back test for Kopia
- extras/test_backup_borg.sh: stop → extract → compare → roll back test for Borg
- backup workers: add START_TS/BACKUP_COUNT/DURATION_STR to completion log and ntfy
- backup workers: pre-flight disk check warns and records failure if < 512 MB free
- services/backup.sh: install test_backup_kopia.sh + optional weekly test timer
- services/borg-backup.sh: install test_backup_borg.sh + optional weekly test timer
- ntfy notifications include service count, duration, and per-failure detail lines