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/prerm

@@ -2,11 +2,15 @@
 # Jarl: runs before the package's files are removed (also on upgrades).
 set -e
 
-# On removal (not on upgrade) stop jarld and remove its boot-time link.
-# named keeps running with the configuration Jarl last wrote.
+# On removal (not on upgrade) stop jarld and the backup timer and remove
+# jarld's boot-time link (postrm deals with the timer's). named keeps
+# running with the configuration Jarl last wrote.
 if [ "$1" = remove ] && [ -d /run/systemd/system ]; then
-	systemctl stop jarl.service 2>/dev/null || true
+	systemctl stop jarl.service jarl-backup.timer jarl-backup.service 2>/dev/null || true
 	systemctl disable jarl.service 2>/dev/null || true
+	if ! command -v deb-systemd-helper >/dev/null 2>&1; then
+		systemctl disable jarl-backup.timer 2>/dev/null || true
+	fi
 fi
 
 exit 0