Disable records instead of deleting them

A disabled record stays in Jarl with its ID, TTL and comment but is
left out of the zone; the zone file lists it as a comment. Web UI
toggle, jarl record disable|enable, record.disable|enable operations
and API endpoints. Database schema 5.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/server/h_registrar.go

@@ -73,7 +73,7 @@ func lowerNoDot(n string) string { return strings.ToLower(strings.TrimSuffix(n,
 // trailing dots.
 func zoneNS(st *model.State, z *model.Zone) []string {
 	var out []string
-	for _, rec := range st.ZoneRecords(z.ID) {
+	for _, rec := range st.ServedRecords(z.ID) {
 		if rec.Type == "NS" && rec.Name == z.Name {
 			out = append(out, lowerNoDot(rec.Data))
 		}