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/client/client.go

@@ -38,6 +38,9 @@ type Client struct {
 	Preflight bool
 	// Mutated records that a preflight run sent a (dry-run) change.
 	Mutated bool
+	// OnTokenExpiry, if set, is called with the token's expiry time when
+	// jarld says the token expires soon (api.TokenExpiresHeader).
+	OnTokenExpiry func(time.Time)
 }
 
 // ErrNoPreflight is returned in Preflight mode for a request that would
@@ -189,6 +192,11 @@ func (c *Client) Do(ctx context.Context, method, path string, query url.Values,
 		return err
 	}
 	defer resp.Body.Close()
+	if v := resp.Header.Get(api.TokenExpiresHeader); v != "" && c.OnTokenExpiry != nil {
+		if t, err := time.Parse(time.RFC3339, v); err == nil {
+			c.OnTokenExpiry(t)
+		}
+	}
 	data, err := io.ReadAll(resp.Body)
 	if err != nil {
 		return err