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-httproute.yaml

@@ -0,0 +1,55 @@
+---
+apiVersion: gateway.networking.k8s.io/v1
+kind: HTTPRoute
+metadata:
+  name: argocd-server-http-redirect
+  namespace: argocd
+  labels:
+    app.kubernetes.io/name: argocd-server
+    app.kubernetes.io/instance: argocd
+spec:
+  hostnames:
+    - argocd.k8s.syd1.au.unkin.net
+  parentRefs:
+    - group: gateway.networking.k8s.io
+      kind: Gateway
+      name: argocd-server
+      sectionName: http
+  rules:
+    - filters:
+        - type: RequestRedirect
+          requestRedirect:
+            scheme: https
+            statusCode: 301
+      matches:
+        - path:
+            type: PathPrefix
+            value: /
+---
+apiVersion: gateway.networking.k8s.io/v1
+kind: HTTPRoute
+metadata:
+  name: argocd-server
+  namespace: argocd
+  labels:
+    app.kubernetes.io/name: argocd-server
+    app.kubernetes.io/instance: argocd
+spec:
+  hostnames:
+    - argocd.k8s.syd1.au.unkin.net
+  parentRefs:
+    - group: gateway.networking.k8s.io
+      kind: Gateway
+      name: argocd-server
+      sectionName: https
+  rules:
+    - backendRefs:
+        - group: ""
+          kind: Service
+          name: argocd-server
+          port: 80
+          weight: 1
+      matches:
+        - path:
+            type: PathPrefix
+            value: /