Fix authoritative secondary replication (TSIG transfer + stable primary)

Secondaries never replicated any member zone: the master's catalog zone
requires key-authenticated AXFR (allow-transfer { key "transfer-key"; }),
but the rendered secondary config transferred without presenting the key,
so every catalog transfer was REFUSED and no member zones provisioned.
Two further gaps compounded it: member zones had no allow-transfer at all,
and secondaries pointed at the primary's pod IP, which dies on restart.

- Render the catalog transfer key into the secondary catalog-zones
  default-primaries and the secondary catalog zone primaries, so
  key-authenticated AXFR from the primary is accepted.
- Add allow-transfer { key "<transfer-key>"; } to catalog member primary
  zones (when the zone does not set an explicit allow-transfer), so
  secondaries can pull them; applied to existing zones via modzone.
- Point secondaries at the stable primary Service ClusterIP instead of the
  primary pod IP, so replication survives primary pod restarts (falls back
  to the pod IP when no primary Service exists).

internal/controller/helpers.go

@@ -108,6 +108,24 @@ func primaryPodIP(ctx context.Context, c client.Client, cluster *bindv1alpha1.Bi
 	return pod.Status.PodIP
 }
 
+// primaryTransferAddress returns the address secondaries use to reach the
+// primary for catalog and zone AXFR. It prefers the primary Service ClusterIP,
+// which is stable across primary pod restarts (the pod IP is not: it changes on
+// every restart, leaving secondaries pointed at a dead address). It falls back
+// to the primary pod IP when no primary Service is configured or its ClusterIP
+// is not yet assigned.
+func primaryTransferAddress(ctx context.Context, c client.Client, cluster *bindv1alpha1.BindCluster) string {
+	if cluster.Spec.PrimaryService != nil {
+		var svc corev1.Service
+		if err := c.Get(ctx, client.ObjectKey{Namespace: cluster.Namespace, Name: primaryServiceName(cluster.Name)}, &svc); err == nil {
+			if ip := svc.Spec.ClusterIP; ip != "" && ip != corev1.ClusterIPNone {
+				return ip
+			}
+		}
+	}
+	return primaryPodIP(ctx, c, cluster)
+}
+
 // resolveTSIG reads the material of a BindTSIGKey into TSIG credentials.
 func resolveTSIG(ctx context.Context, c client.Client, namespace, keyRef string) (bind.TSIGCreds, error) {
 	var creds bind.TSIGCreds