Mount the rancher secrets engine + seed a service account + roles (#93)

## Why

Deploy the new Rancher token secrets engine into the cluster (the last of the 4 wiring PRs), mirroring the litellm/gpg pattern. Users can then `vault read rancher/creds/<role>` for short-lived, cluster-scoped Rancher tokens, backed by a seeded admin token the engine auto-rotates before Rancher's 90-day cap.

## Changes

- 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); the seed token is read from KV, not git.
- Add `rancher_secret_backend_role` module + a `ci` role (1h/8h, cluster+TTL scoped).
- Wire `config.hcl` discovery, module variables, `main.tf` blocks, terragrunt inputs, and the `rancher` provider in `root.hcl`.
- Config: `config/rancher_secret_backend/rancher.yaml`, `.../service_account/rancher/admin.yaml`, `.../role/rancher/ci.yaml`.

## Prerequisite

Populate `kv/service/vault/au/syd1/secret_backend/rancher/service_account/admin` with a live Rancher admin token (keys: `token`, optional `token_name`) **before** apply, exactly as litellm's `master_key` is seeded in KV.

## Merge order

Part 4 of 4 (last). Requires: puppet install (#483) → deployer policy (#91) → plugin import (#92) → this. The `plan` needs the KV seed present, so seed KV first.

---------

Co-authored-by: Ben Vincent <neotheo@gmail.com>
Reviewed-on: #93
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

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"