Add auth and state access for terraform-authentik (#78)

## Summary
- K8s auth role for Woodpecker CI (`terraform-authentik` SA in `woodpecker` namespace)
- AppRole for local terraform runs
- Consul secret backend role (`terraform-authentik`, TTL 120/300)
- Consul ACL policy for `infra/terraform/authentik/` key prefix
- Vault policy granting both auth methods access to Consul creds

Reviewed-on: #78
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

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

@@ -0,0 +1,11 @@
+---
+rules:
+  - path: "consul_root/au/syd1/creds/terraform-authentik"
+    capabilities:
+      - read
+
+auth:
+  approle:
+    - terraform_authentik
+  k8s/au/syd1:
+    - woodpecker_terraform_authentik