Add ArgoCD OAuth2/OIDC provider

Extends Authentik SSO to ArgoCD so cluster operators log in with their
Authentik identity and group membership instead of the local admin account.

- Add config/providers_oauth2/argocd.yaml: confidential OAuth2 provider +
  application (slug argocd), client_id argocd, openid/email/profile scopes,
  web SSO and CLI (localhost:8085) redirect URIs. client_secret is read from
  Vault at kv/kubernetes/namespace/argocd/default/oauth-credentials, matching
  the existing Grafana pattern.

config/providers_oauth2/argocd.yaml

@@ -0,0 +1,21 @@
+# OAuth2/OIDC provider + application for the in-cluster ArgoCD
+# (argocd.k8s.syd1.au.unkin.net). client_secret is read from Vault, not committed.
+name: ArgoCD
+authorization_flow: default-provider-authorization-implicit-consent
+invalidation_flow: default-provider-invalidation-flow
+client_type: confidential
+client_id: argocd
+client_secret_vault:
+  mount: kv
+  path: kubernetes/namespace/argocd/default/oauth-credentials
+scope_mappings:
+  - goauthentik.io/providers/oauth2/scope-openid
+  - goauthentik.io/providers/oauth2/scope-email
+  - goauthentik.io/providers/oauth2/scope-profile
+redirect_uris:
+  # Web UI SSO callback.
+  - matching_mode: strict
+    url: https://argocd.k8s.syd1.au.unkin.net/auth/callback
+  # `argocd login --sso` CLI callback (local listener).
+  - matching_mode: strict
+    url: http://localhost:8085/auth/callback