Mount the rancher secrets engine + seed a service account + roles

Deploys the Rancher token secrets engine into the cluster, mirroring the litellm
and gpg wiring. The plugin is registered separately (config/plugins) and the
deployer policy is granted separately; this mounts and configures the engine.

- Add rancher_secret_backend module (mount + config via the ranchervaultsecret
  provider): rancher_url https://rancher.k8s.syd1.au.unkin.net.
- Add rancher_secret_backend_service_account module: seeds an auto-rotated token
  (90d TTL / 45d rotation), reading the seed token from KV at
  secret_backend/rancher/service_account/<name> (keys: token, token_name).
- Add rancher_secret_backend_role module + a 'ci' role (1h/8h scoped creds).
- Wire config.hcl discovery, module variables, main.tf module blocks, the
  terragrunt inputs, and the rancher provider in root.hcl.

Prereq: populate kv/service/vault/au/syd1/secret_backend/rancher/service_account/admin
with a live Rancher admin token before apply.

modules/vault_cluster/main.tf

@@ -374,6 +374,53 @@ module "gpg_key" {
   depends_on = [module.gpg_secret_backend]
 }
 
+module "rancher_secret_backend" {
+  source = "./modules/rancher_secret_backend"
+
+  for_each = var.rancher_secret_backend
+
+  path                    = each.key
+  plugin                  = each.value.plugin
+  description             = each.value.description
+  rancher_url             = each.value.rancher_url
+  ca_cert                 = each.value.ca_cert
+  tls_skip_verify         = each.value.tls_skip_verify
+  request_timeout_seconds = each.value.request_timeout_seconds
+
+  depends_on = [module.plugin]
+}
+
+module "rancher_secret_backend_service_account" {
+  source = "./modules/rancher_secret_backend_service_account"
+
+  for_each = var.rancher_secret_backend_service_account
+
+  backend         = each.value.backend
+  name            = each.value.name
+  country         = var.country
+  region          = var.region
+  token_ttl       = each.value.token_ttl
+  rotation_period = each.value.rotation_period
+
+  depends_on = [module.rancher_secret_backend]
+}
+
+module "rancher_secret_backend_role" {
+  source = "./modules/rancher_secret_backend_role"
+
+  for_each = var.rancher_secret_backend_role
+
+  backend         = each.value.backend
+  name            = each.value.name
+  service_account = each.value.service_account
+  cluster_name    = each.value.cluster_name
+  description     = each.value.description
+  ttl             = each.value.ttl
+  max_ttl         = each.value.max_ttl
+
+  depends_on = [module.rancher_secret_backend_service_account]
+}
+
 module "vault_policy" {
   source = "./modules/vault_policy"