Add Rancher OAuth2/OIDC provider

Extends Authentik SSO to Rancher (rancher.k8s.syd1.au.unkin.net) so cluster
access uses Authentik identity and groups.

- Add config/providers_oauth2/rancher.yaml: confidential OAuth2 provider +
  application (slug rancher), openid/email/profile scopes, /verify-auth
  redirect (Rancher's OIDC callback). client_secret read from Vault at
  kv/kubernetes/namespace/cattle-system/default/oauth-credentials.

config/providers_oauth2/rancher.yaml

@@ -0,0 +1,18 @@
+# OAuth2/OIDC provider + application for Rancher
+# (rancher.k8s.syd1.au.unkin.net). client_secret is read from Vault, not committed.
+# Rancher's OIDC/Keycloak auth provider redirects back to /verify-auth.
+name: Rancher
+authorization_flow: default-provider-authorization-implicit-consent
+invalidation_flow: default-provider-invalidation-flow
+client_type: confidential
+client_id: rancher
+client_secret_vault:
+  mount: kv
+  path: kubernetes/namespace/cattle-system/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:
+  - matching_mode: strict
+    url: https://rancher.k8s.syd1.au.unkin.net/verify-auth