Add backups, token expiry warnings and name server checks

- Daily database backups: jarl-backup.timer runs 'jarld backup', which
  writes jarl-<UTC time>.db into backup.dir, checks it before naming it,
  and prunes by the time in the name while always keeping the newest
  keep_at_least. New backup: section in jarl.yaml; the dashboard and
  'jarl status' show the newest backup and warn when it is stale or
  failed. The Debian package enables the timer.
- 'jarld restore [FILE | latest]': checks the backup, shows it next to
  the current database, keeps the current one aside and records the
  restore. 'jarld serve' holds a database lock, so a restore can't run
  under a live jarld and two jarlds can't share a database.
- Token expiry warnings from 14 days ahead: dashboard, token lists, a
  daily log line, and a CLI warning via the Jarl-Token-Expires header;
  'jarl token list --expiring'.
- 'jarl zone serials' and the zone page's Name servers tab: compare the
  serial on a primary zone's secondaries (NS records, also-notify) or a
  secondary zone's primaries with this server's.
- 'jarl zone delegation': for zones on the internet, check the parent's
  delegation and glue against the zone; an undelegated zone gets a note.
- Server: Jarl/<version> header; CLI exit code 2 for failed checks.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

deploy/debian/postrm

@@ -10,12 +10,27 @@ remove|purge)
 	;;
 esac
 
+# The backup timer's enabled state (see postinst): masked while the
+# package is removed, forgotten on purge.
+if command -v deb-systemd-helper >/dev/null 2>&1; then
+	case "$1" in
+	remove)
+		deb-systemd-helper mask jarl-backup.timer >/dev/null || true
+		;;
+	purge)
+		deb-systemd-helper purge jarl-backup.timer >/dev/null || true
+		deb-systemd-helper unmask jarl-backup.timer >/dev/null || true
+		;;
+	esac
+fi
+
 # Even a purge keeps Jarl's data: the database in /var/lib/jarl is the
 # source of your DNS configuration, and named.conf may still include the
 # files in /etc/bind/jarl. Remove them by hand when you no longer need them.
 if [ "$1" = purge ]; then
-	echo "jarl: kept /etc/jarl, /var/lib/jarl, /etc/bind/jarl, /var/lib/bind/jarl and"
-	echo "/var/cache/bind/jarl (named may still use them); remove them by hand if unused."
+	echo "jarl: kept /etc/jarl, /var/lib/jarl, /var/backups/jarl, /etc/bind/jarl,"
+	echo "/var/lib/bind/jarl and /var/cache/bind/jarl (named may still use some of"
+	echo "them); remove them by hand if unused."
 fi
 
 exit 0