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>

internal/engine/engine.go

@@ -1190,3 +1190,17 @@ func (e *Engine) WorkingState(ctx context.Context, csID int64, p *auth.Principal
 	}
 	return work, base, nil
 }
+
+// LoadedSerial returns the serial named serves for zone z, from rndc
+// zonestatus. ok is false when named isn't asked (no_reload) or doesn't
+// report a serial (the zone isn't loaded yet).
+func (e *Engine) LoadedSerial(ctx context.Context, st *model.State, z *model.Zone) (serial uint32, ok bool, err error) {
+	if e.cfg.NoReload || e.tools == nil {
+		return 0, false, nil
+	}
+	zs, err := e.tools.ZoneStatus(ctx, z.Name, e.namedView(st, z))
+	if err != nil {
+		return 0, false, err
+	}
+	return zs.Serial, zs.HasSerial, nil
+}