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/au/syd1/terragrunt.hcl

@@ -74,6 +74,10 @@ inputs = {
   gpg_secret_backend             = local.config.gpg_secret_backend
   gpg_key                        = local.config.gpg_key
 
+  rancher_secret_backend                 = local.config.rancher_secret_backend
+  rancher_secret_backend_service_account = local.config.rancher_secret_backend_service_account
+  rancher_secret_backend_role            = local.config.rancher_secret_backend_role
+
   # Pass policy maps to vault_cluster module
   policy_auth_map  = local.policies.policy_auth_map
   policy_rules_map = local.policies.policy_rules_map