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.

environments/root.hcl

@@ -23,6 +23,12 @@ provider "gpg" {
   address = local.vault_addr
 }
 
+# The rancher token secrets engine is managed through its own provider (same
+# Vault server; token falls back to VAULT_TOKEN).
+provider "rancher" {
+  address = local.vault_addr
+}
+
 terraform {
   backend "consul" {
     address = "https://consul.service.consul"
@@ -49,6 +55,10 @@ terraform {
       source  = "artifactapi.k8s.syd1.au.unkin.net/terraform-unkin/gpgvaultsecret"
       version = "0.1.0"
     }
+    rancher = {
+      source  = "artifactapi.k8s.syd1.au.unkin.net/terraform-unkin/ranchervaultsecret"
+      version = "0.1.0"
+    }
   }
 }
 EOF