arrstack: manage sonarr/radarr/prowlarr API keys via Vault (#369)

## Why

The sonarr/radarr/prowlarr images self-generate an API key into
/config/config.xml on first boot, so the key is unmanaged and differs per
volume reset. This makes Vault the source of truth for those keys (override
bootstrap, chosen by Ben): the key is minted in Vault and enforced into
config.xml before each app starts.

## Changes

- Add a `VaultAuth` `default` in the `arrstack` namespace (kubernetes auth,
  mount `k8s/au/syd1`, role `default`, SA `default`), mirroring jellyfin.
- Add a per-app `VaultStaticSecret` that syncs
  `kv/kubernetes/namespace/arrstack/default/<app>` (key `apitoken`) into the
  `<app>-apikey` Secret. The `default` k8s role's templated policy already
  grants read on that path for the `arrstack/default` SA, so no
  terraform-vault change is needed.
- Add an `apikey-init` initContainer to each of the three deployments that
  reads `API_KEY` from the VSO-created Secret, fails closed on a missing or
  non-hex value, and writes/updates only the `<ApiKey>` element in
  `/config/config.xml` (then fixes ownership 1000:1000, mode 600). Image is a
  pinned busybox via artifactapi to keep this PR atomic (no new image
  dependency).
- Wire the new manifests into the base and per-app kustomizations.

## Notes

- Keys already seeded in Vault at `kv/kubernetes/namespace/arrstack/default/<app>`.
- nzbget is out of scope: it has no config.xml `<ApiKey>` (uses ControlPassword),
  a separate follow-up.
- Downstream consumers (proxy, terraform) currently read
  `kv/service/media-apps/<app>`; the authoritative key now lives at the path
  above. Reconciliation is deferred.

---------

Co-authored-by: unkin-agent <unkin-agent@git.unkin.net>
Reviewed-on: #369
Co-authored-by: Unkin Agent <unkin-agent@unkin.net>
Co-committed-by: Unkin Agent <unkin-agent@unkin.net>

apps/base/arrstack/vaultauth.yaml

@@ -0,0 +1,20 @@
+---
+apiVersion: secrets.hashicorp.com/v1beta1
+kind: VaultAuth
+metadata:
+  name: default
+  namespace: arrstack
+  annotations:
+    argocd.argoproj.io/sync-wave: "0"
+spec:
+  allowedNamespaces:
+    - arrstack
+  kubernetes:
+    audiences:
+      - vault
+    role: default
+    serviceAccount: default
+    tokenExpirationSeconds: 600
+  method: kubernetes
+  mount: k8s/au/syd1
+  vaultConnectionRef: vso-system/default