Wire ArgoCD to Authentik OIDC

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 identity and group membership.

- argocd-cm: set url and oidc.config (Authentik issuer, argocd client,
  openid/profile/email scopes); client secret resolved from the argocd-oidc
  Secret via $argocd-oidc:client_secret
- argocd-rbac-cm: match 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 grafana)
- 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)

Note: the OIDC client secret must be seeded in Vault out of band, and
argocd-server needs a one-time rollout restart to pick up server.insecure.

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