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/server/routes.go

@@ -65,6 +65,8 @@ func (s *Server) routes() {
 	m.HandleFunc("GET "+p+"/zones/{zone}", s.authed(s.getZone))
 	m.HandleFunc("PATCH "+p+"/zones/{zone}", s.authed(s.updateZone))
 	m.HandleFunc("DELETE "+p+"/zones/{zone}", s.authed(s.deleteZone))
+	m.HandleFunc("GET "+p+"/zones/{zone}/serials", s.authed(s.zoneSerials))
+	m.HandleFunc("GET "+p+"/zones/{zone}/delegation", s.authed(s.zoneDelegation))
 	m.HandleFunc("GET "+p+"/zones/{zone}/file", s.authed(s.zoneFile))
 	m.HandleFunc("POST "+p+"/zones/{zone}/import", s.authed(s.importZoneFile))
 	m.HandleFunc("GET "+p+"/zones/{zone}/records", s.authed(s.listRecords))