Add auth and state access for terraform-rancher (#86)

## Why

The new `terraform-rancher` repo (manages Rancher's Authentik OIDC auth via the rancher2 provider) needs Vault auth + Consul state, mirroring the terraform-authentik runner (#78/#81/#82).

## Change

- `AppRole/terraform_rancher` + k8s auth role `woodpecker_terraform_rancher` (SA terraform-rancher in the woodpecker ns).
- Consul secret-backend role + ACL policy (`resources/secret_backend/consul_root/au/syd1/terraform-rancher.hcl`) granting write to the `infra/terraform/rancher/` state prefix.
- Vault policies: read the Rancher admin API token (`kv/service/terraform/rancher`) and the keycloakoidc client secret (`kv/kubernetes/namespace/cattle-system/default/oauth-credentials`), plus the consul_root state creds.

Scoped the OAuth read to the `cattle-system` path specifically (rather than the `+` wildcard the authentik policy uses) since the Rancher runner only needs its own app's secret.

## Validation

pre-commit (terragrunt-hcl-fmt + yamllint) passed. CI plan will confirm.

Reviewed-on: #86
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

policies/consul_root/au/syd1/creds/terraform-rancher.yaml

@@ -0,0 +1,11 @@
+---
+rules:
+  - path: "consul_root/au/syd1/creds/terraform-rancher"
+    capabilities:
+      - read
+
+auth:
+  approle:
+    - terraform_rancher
+  k8s/au/syd1:
+    - woodpecker_terraform_rancher