Scaffold terraform-rancher: Authentik OIDC auth config

Manages Rancher's Keycloak(OIDC) auth provider via the rancher2 provider,
pointed at Authentik. Mirrors the terraform-authentik layout (terragrunt +
Vault-sourced secrets + Woodpecker plan/apply/pre-commit pipelines).

- modules/rancher: rancher2_auth_config_keycloak_oidc, client_secret read from
  Vault (kv/kubernetes/namespace/cattle-system/default/oauth-credentials);
  access_mode unrestricted to avoid admin lockout on enable.
- config/keycloakoidc.yaml: issuer/auth_endpoint at identity.unkin.net,
  client_id rancher, /verify-auth redirect, openid/profile/email scopes.
- environments/rancher.k8s.syd1.au.unkin.net: consul state at
  infra/terraform/rancher/, rancher2 provider api_url from the env name.
- rancher2 admin token read from kv/service/terraform/rancher (Makefile);
  to migrate to a dedicated Vault Rancher secrets engine (90-day token cap).

Validated with `tofu validate` (config valid against the rancher2 provider).
A live `plan` needs the Rancher admin API token seeded in Vault first.

.woodpecker/pre-commit.yaml

@@ -0,0 +1,18 @@
+when:
+  - event: pull_request
+
+steps:
+  - name: pre-commit
+    image: git.unkin.net/unkin/almalinux9-opentofu:20260606
+    commands:
+      - uvx pre-commit run --all-files
+    backend_options:
+      kubernetes:
+        serviceAccountName: default
+        resources:
+          requests:
+            memory: 512Mi
+            cpu: 1
+          limits:
+            memory: 2Gi
+            cpu: 2