Part of the bind rollout split. **Merge #219 (bind-operator) first** — this PR is stacked on it, so its diff will reduce to just the binddns-auth files once #219 merges.
## Why
The authoritative masters tier (replaces 3x Puppet authoritative servers): pod-0 primary + 2 secondaries replicating via the catalog zone + AXFR/IXFR.
## Changes
- `apps/base/binddns-auth`: authoritative `BindCluster` (3 replicas, LoadBalancer/PureLB), `BindCatalogZone`, transfer `BindTSIGKey`, namespace
- au-syd1 `binddns-auth` overlay
## Deploy impact
Creates the `binddns-auth` StatefulSet + LoadBalancer once merged.
Reviewed-on: #220
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>
