Scaffold terraform-rancher: Authentik OIDC auth config

Manages Rancher's Keycloak(OIDC) auth provider via the rancher2 provider,
pointed at Authentik. Mirrors the terraform-authentik layout (terragrunt +
Vault-sourced secrets + Woodpecker plan/apply/pre-commit pipelines).

- modules/rancher: rancher2_auth_config_keycloak_oidc, client_secret read from
  Vault (kv/kubernetes/namespace/cattle-system/default/oauth-credentials);
  access_mode unrestricted to avoid admin lockout on enable.
- config/keycloakoidc.yaml: issuer/auth_endpoint at identity.unkin.net,
  client_id rancher, /verify-auth redirect, openid/profile/email scopes.
- environments/rancher.k8s.syd1.au.unkin.net: consul state at
  infra/terraform/rancher/, rancher2 provider api_url from the env name.
- rancher2 admin token read from kv/service/terraform/rancher (Makefile);
  to migrate to a dedicated Vault Rancher secrets engine (90-day token cap).

Validated with `tofu validate` (config valid against the rancher2 provider).
A live `plan` needs the Rancher admin API token seeded in Vault first.

environments/rancher.k8s.syd1.au.unkin.net/terragrunt.hcl

@@ -0,0 +1,21 @@
+include "root" {
+  path   = find_in_parent_folders("root.hcl")
+  expose = true
+}
+
+include "config" {
+  path   = "${get_repo_root()}/config/config.hcl"
+  expose = true
+}
+
+locals {
+  config = include.config.locals.config
+}
+
+terraform {
+  source = "../../modules/rancher"
+}
+
+inputs = {
+  keycloakoidc = local.config.keycloakoidc
+}