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-server-gateway.yaml

@@ -0,0 +1,39 @@
+---
+apiVersion: gateway.networking.k8s.io/v1
+kind: Gateway
+metadata:
+  name: argocd-server
+  namespace: argocd
+  labels:
+    app.kubernetes.io/name: argocd-server
+    app.kubernetes.io/instance: argocd
+    traefik.io/instance: internal
+  annotations:
+    cert-manager.io/cluster-issuer: vault-issuer
+    cert-manager.io/common-name: argocd.k8s.syd1.au.unkin.net
+    cert-manager.io/private-key-size: "4096"
+    external-dns.alpha.kubernetes.io/hostname: argocd.k8s.syd1.au.unkin.net
+    external-dns.alpha.kubernetes.io/target: 198.18.200.4
+spec:
+  gatewayClassName: traefik-internal
+  listeners:
+    - name: http
+      port: 80
+      protocol: HTTP
+      hostname: argocd.k8s.syd1.au.unkin.net
+      allowedRoutes:
+        namespaces:
+          from: Same
+    - name: https
+      port: 443
+      protocol: HTTPS
+      hostname: argocd.k8s.syd1.au.unkin.net
+      allowedRoutes:
+        namespaces:
+          from: Same
+      tls:
+        mode: Terminate
+        certificateRefs:
+          - group: ""
+            kind: Secret
+            name: argocd-server-tls