ops: add nightly Docker volume backup to SAMURAI

- scripts/backup-volumes.sh: tar each named volume via alpine, rsync to
  SAMURAI (Tailscale 100.74.x.x) at 02:00; 7-day retention; preflight
  checks Tailscale + SSH before starting
- scripts/setup-samurai-ssh.sh: one-time SSH key install to SAMURAI
- scripts/monk-backup.{service,timer}: systemd units for nightly schedule
- docs/backup-setup.md: full setup instructions incl. Windows OpenSSH
  config and admin authorized_keys fix

Phase 2 (MinIO S3 on SAMURAI) tracked as TODO in backup-volumes.sh.

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

scripts/monk-backup.timer

@@ -0,0 +1,10 @@
+[Unit]
+Description=Nightly KiteStacks Docker volume backup at 02:00
+
+[Timer]
+OnCalendar=*-*-* 02:00:00
+Persistent=true
+RandomizedDelaySec=300
+
+[Install]
+WantedBy=timers.target