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-cm-patch.yaml

@@ -6,3 +6,20 @@ metadata:
   namespace: argocd
 data:
   kustomize.buildOptions: "--enable-helm"
+  # External URL ArgoCD serves on (TLS terminated at the traefik-internal gateway).
+  url: https://argocd.k8s.syd1.au.unkin.net
+  # OIDC login via Authentik. The client secret is seeded in Vault out of band
+  # and surfaced as the `argocd-oidc` Secret (labelled part-of=argocd) by VSO;
+  # `$argocd-oidc:client_secret` resolves the key from that Secret.
+  oidc.config: |
+    name: Authentik
+    issuer: https://identity.unkin.net/application/o/argocd/
+    clientID: argocd
+    clientSecret: $argocd-oidc:client_secret
+    requestedScopes:
+      - openid
+      - profile
+      - email
+    requestedIDTokenClaims:
+      groups:
+        essential: true