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/variables.tf

@@ -350,6 +350,44 @@ variable "gpg_key" {
   default = {}
 }
 
+variable "rancher_secret_backend" {
+  description = "Map of rancher token secret engines to create (mount + config)"
+  type = map(object({
+    plugin                  = optional(string, "vault-plugin-secrets-rancher")
+    description             = optional(string)
+    rancher_url             = string
+    ca_cert                 = optional(string)
+    tls_skip_verify         = optional(bool, false)
+    request_timeout_seconds = optional(number, 30)
+  }))
+  default = {}
+}
+
+variable "rancher_secret_backend_service_account" {
+  description = "Map of seeded, auto-rotated rancher service-account tokens (seed token read from KV)"
+  type = map(object({
+    name            = string
+    backend         = string
+    token_ttl       = optional(number)
+    rotation_period = optional(number)
+  }))
+  default = {}
+}
+
+variable "rancher_secret_backend_role" {
+  description = "Map of rancher token-minting roles to create"
+  type = map(object({
+    name            = string
+    backend         = string
+    service_account = string
+    cluster_name    = optional(string)
+    description     = optional(string)
+    ttl             = optional(number)
+    max_ttl         = optional(number)
+  }))
+  default = {}
+}
+
 variable "policy_auth_map" {
   description = "Map of auth mounts -> auth roles -> policy names"
   type        = map(map(list(string)))