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>

cmd/jarld/admin.go

@@ -356,30 +356,6 @@ func resyncCmd() *cobra.Command {
 	return cmd
 }
 
-func backupCmd() *cobra.Command {
-	return &cobra.Command{
-		Use:   "backup FILE",
-		Short: "Write a consistent copy of the database to FILE (safe while jarld runs)",
-		Args:  cobra.ExactArgs(1),
-		RunE: func(cmd *cobra.Command, args []string) error {
-			a, err := openApp(true)
-			if err != nil {
-				return err
-			}
-			defer a.close()
-			if _, err := os.Stat(args[0]); err == nil {
-				return fmt.Errorf("%s already exists", args[0])
-			}
-			if err := a.store.Backup(context.Background(), args[0]); err != nil {
-				return err
-			}
-			os.Chmod(args[0], 0o600)
-			fmt.Println("Database copied to", args[0])
-			return nil
-		},
-	}
-}
-
 func configCmd() *cobra.Command {
 	var example bool
 	cmd := &cobra.Command{