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>

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