Wire ArgoCD to Authentik OIDC (#253)

## Why

ArgoCD had no external ingress and only local admin auth. This exposes `argocd-server` behind the traefik-internal gateway and enables Authentik SSO, so operators log in with their Authentik identity and group membership. Pairs with unkin/terraform-authentik#3 (creates the OAuth2 provider).

## Changes

- **argocd-cm**: set `url` and `oidc.config` (Authentik issuer `identity.unkin.net/application/o/argocd/`, `argocd` client, openid/profile/email scopes). Client secret resolved from the `argocd-oidc` Secret via `$argocd-oidc:client_secret`.
- **argocd-rbac-cm**: match RBAC on the `groups` claim; default `role:readonly`; map the `argocd-admins` Authentik group to `role:admin`.
- **argocd-cmd-params-cm**: `server.insecure=true` so `argocd-server` serves HTTP behind the TLS-terminating gateway.
- Add **Gateway + HTTPRoutes** for `argocd.k8s.syd1.au.unkin.net` (mirrors the grafana pattern: traefik-internal, vault-issuer cert, external-dns).
- Add **VaultAuth + VaultStaticSecret** sourcing the OIDC client secret from `kv/kubernetes/namespace/argocd/default/oauth-credentials` into the `argocd-oidc` Secret (labelled `part-of=argocd` so ArgoCD will resolve the `$` reference).

## Notes / rollout

- Seed the client secret in Vault out of band (same path terraform-authentik reads).
- The argocd namespace `default` SA already has Vault read access via the `default` k8s role, so no terraform-vault change is needed.
- `argocd-server` needs a one-time rollout restart to pick up `server.insecure`.

Validated with `kustomize build --enable-helm clusters/au-syd1/bootstrap`, `make kubeconform`, and pre-commit (yamllint + no-plain-secrets).

Reviewed-on: #253
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

clusters/au-syd1/bootstrap/argocd-oidc-vaultstaticsecret.yaml

@@ -0,0 +1,23 @@
+---
+apiVersion: secrets.hashicorp.com/v1beta1
+kind: VaultStaticSecret
+metadata:
+  name: argocd-oidc
+  namespace: argocd
+spec:
+  vaultAuthRef: default
+  mount: kv
+  type: kv-v2
+  # Seeded out of band; same secret is read by terraform-authentik to configure
+  # the provider's client_secret. Key: client_secret.
+  path: kubernetes/namespace/argocd/default/oauth-credentials
+  refreshAfter: 5m
+  hmacSecretData: true
+  destination:
+    name: argocd-oidc
+    create: true
+    overwrite: true
+    # ArgoCD only resolves `$secret:key` references against Secrets carrying
+    # this label.
+    labels:
+      app.kubernetes.io/part-of: argocd