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>

config/config.hcl

@@ -218,5 +218,26 @@ locals {
       })
       if startswith(file_path, "gpg_key/")
     }
+    rancher_secret_backend = {
+      for file_path, content in local.all_configs :
+      trimsuffix(basename(file_path), ".yaml") => content
+      if startswith(file_path, "rancher_secret_backend/")
+    }
+    rancher_secret_backend_service_account = {
+      for file_path, content in local.all_configs :
+      trimsuffix(replace(file_path, "rancher_secret_backend_service_account/", ""), ".yaml") => merge(content, {
+        name    = trimsuffix(basename(file_path), ".yaml")
+        backend = dirname(replace(file_path, "rancher_secret_backend_service_account/", ""))
+      })
+      if startswith(file_path, "rancher_secret_backend_service_account/")
+    }
+    rancher_secret_backend_role = {
+      for file_path, content in local.all_configs :
+      trimsuffix(replace(file_path, "rancher_secret_backend_role/", ""), ".yaml") => merge(content, {
+        name    = trimsuffix(basename(file_path), ".yaml")
+        backend = dirname(replace(file_path, "rancher_secret_backend_role/", ""))
+      })
+      if startswith(file_path, "rancher_secret_backend_role/")
+    }
   }
 }