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/model/changeset.go

@@ -97,6 +97,16 @@ type Token struct {
 	PrevExpiresAt  *time.Time `json:"previous_secret_valid_until,omitempty"`
 }
 
+// TokenExpiryWarning is how long before a token expires Jarl starts to
+// warn about it: on the dashboard, in jarld's log, and to the CLI using it.
+const TokenExpiryWarning = 14 * 24 * time.Hour
+
+// ExpiresWithin reports whether the token is still in use (not revoked)
+// and expires within d from now. A token that has already expired counts.
+func (t *Token) ExpiresWithin(now time.Time, d time.Duration) bool {
+	return t.RevokedAt == nil && t.ExpiresAt != nil && t.ExpiresAt.Sub(now) < d
+}
+
 // IssuedAt is when the token's current secret was made.
 func (t *Token) IssuedAt() time.Time {
 	if t.RotatedAt != nil {