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-system/kustomization.yaml

@@ -6,6 +6,6 @@ resources:
   - namespace.yaml
   # CRDs are pulled from the bind-operator repo at the matching tag rather than
   # vendored here, so they never drift from the operator.
-  - https://git.unkin.net/unkin/bind-operator/raw/tag/v0.1.1/config/crd/install.yaml
+  - https://git.unkin.net/unkin/bind-operator/raw/tag/v0.1.3/config/crd/install.yaml
   - rbac.yaml
   - deployment.yaml