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_role/rancher/ci.yaml

@@ -0,0 +1,9 @@
+# A role that mints short-lived Rancher tokens from the "admin" service account.
+# Reading rancher/creds/ci returns a lease-bound token deleted from Rancher on
+# revoke. Minted tokens inherit the admin service account's RBAC; only cluster
+# and TTL are scoped per-token.
+---
+service_account: admin
+description: "CI/CD ephemeral Rancher token"
+ttl: 3600      # seconds (1h)
+max_ttl: 28800 # seconds (8h)