Consolidate BIND DNS into one bind-internal namespace (#225)

**HOLD until v0.1.3 is tagged/built** (operator #4 merged + tagged) — this PR bumps the operator to v0.1.3, whose CRD adds the `clusterRef` field these keys use.

## Why
Put all BIND DNS services in one `bind-internal` namespace and name the StatefulSets clearly.

## Changes
- 3 clusters consolidated into `bind-internal`, StatefulSets renamed **bind-authoritative** / **bind-resolvers** / **bind-externaldns**; LBs kept on 198.18.200.6/.7/.8; external-dns hostnames renamed to match
- `clusterRef` added to `transfer-key` (→ bind-authoritative) and `externaldns-key` (→ bind-externaldns) so keys are scoped per cluster
- removed the old `ns-auth`/`ns-resolver`/`ns-externaldns` apps; ApplicationSet + AppProject now list `bind-internal`
- bumped `bind-system` operator to **v0.1.3** (CRD link + image)
- operator stays in `bind-system`

## Deploy impact
ArgoCD prunes the old ns-* namespaces (StatefulSets/PVCs — data is only seed SOA+NS, no migrated records yet) and creates the renamed clusters in bind-internal.

## Validated
`kustomize build` → 28 docs (3 BindCluster, 20 BindZone, 2 catalog, 2 keys, ns); kubeconform clean.

Reviewed-on: #225
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

apps/base/bind-internal/externaldns/tsigkey.yaml

@@ -0,0 +1,11 @@
+---
+# Key that external-dns (and DNSRecord objects) use to send RFC2136 dynamic
+# updates to the primary. The operator generates the material into a Secret.
+apiVersion: bind.unkin.net/v1alpha1
+kind: BindTSIGKey
+metadata:
+  name: externaldns-key
+  namespace: bind-internal
+spec:
+  clusterRef: bind-externaldns
+  algorithm: hmac-sha256