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/kustomization.yaml

@@ -7,12 +7,25 @@ resources:
   - https://artifactapi.k8s.syd1.au.unkin.net/api/v1/remote/github/kubernetes-sigs/gateway-api/releases/download/v1.5.1/standard-install.yaml
   - au-syd1-apps.yaml
   - argocd-self-app.yaml
+  # Authentik OIDC: expose argocd-server and wire the client secret from Vault.
+  - argocd-server-gateway.yaml
+  - argocd-server-httproute.yaml
+  - argocd-oidc-vaultauth.yaml
+  - argocd-oidc-vaultstaticsecret.yaml
 
 patches:
   - path: argocd-cm-patch.yaml
     target:
       kind: ConfigMap
       name: argocd-cm
+  - path: argocd-rbac-cm-patch.yaml
+    target:
+      kind: ConfigMap
+      name: argocd-rbac-cm
+  - path: argocd-cmd-params-cm-patch.yaml
+    target:
+      kind: ConfigMap
+      name: argocd-cmd-params-cm
   - path: argocd-tls-certs-patch.yaml
     target:
       kind: ConfigMap