Add LiteLLM OAuth2/OIDC provider

SSO for LiteLLM (litellm.k8s.syd1.au.unkin.net) via Authentik generic OIDC.
Confidential OAuth2 provider + application, openid/email/profile scopes,
/sso/callback redirect. client_secret read from Vault at
kv/kubernetes/namespace/litellm/default/oauth-credentials (seeded).

config/providers_oauth2/litellm.yaml

@@ -0,0 +1,18 @@
+# OAuth2/OIDC provider + application for LiteLLM
+# (litellm.k8s.syd1.au.unkin.net). client_secret is read from Vault, not committed.
+# LiteLLM's generic SSO redirects back to /sso/callback.
+name: LiteLLM
+authorization_flow: default-provider-authorization-implicit-consent
+invalidation_flow: default-provider-invalidation-flow
+client_type: confidential
+client_id: litellm
+client_secret_vault:
+  mount: kv
+  path: kubernetes/namespace/litellm/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://litellm.k8s.syd1.au.unkin.net/sso/callback