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.

config/rancher_secret_backend/rancher.yaml

@@ -0,0 +1,8 @@
+# Mounts the rancher token secrets engine at "rancher" and writes its config.
+# The plugin is registered in the catalog separately (see
+# config/plugins/vault-plugin-secrets-rancher.yaml). Seeded service-account
+# tokens live under config/rancher_secret_backend_service_account/rancher/ and
+# roles under config/rancher_secret_backend_role/rancher/.
+description: "Rancher API token engine (seeded root rotation + dynamic scoped creds)"
+rancher_url: "https://rancher.k8s.syd1.au.unkin.net"
+request_timeout_seconds: 30