Add Authentik OIDC SSO as the default human login for OpenBao (#147)

## Why

Human login to OpenBao is LDAP-only, so operators keep a second credential set outside Authentik and group membership is maintained twice.

## How

- Add `auth_oidc_backend` module: `oidc`-type JWT auth mount, Authentik discovery URL, `listing_visibility: unauth`, credentials from `kv/service/authentik/oidc-vault`.
- Add `auth_oidc_role` module: oidc role with `user_claim` email, `groups_claim` `ak_groups`, scopes `openid profile email ak_groups`, the registered redirect URIs, `bound_audiences` `[vault]`.
- Add `auth_oidc_group` module: external identity group plus group alias on the OIDC mount accessor, policies from `policy_auth_map`.
- Add config under `config/auth_oidc_{backend,role,group}/`, discovery locals in `config/config.hcl`, `vault_cluster` variables and wiring, and terragrunt inputs.
- Bind `akP-vault-admin` on the `oidc` mount to `global-root`, alongside the existing LDAP `vault_admin` binding.
- Pin the mount path to the literal `oidc`: the registered redirect URIs embed `/ui/vault/auth/oidc/oidc/callback`.

Requires #146, terraform-authentik #33 and #148 applied first.

---------

Co-authored-by: BenVincent <benvin@main.unkin.net>
Reviewed-on: #147
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

modules/vault_cluster/main.tf

@@ -64,6 +64,55 @@ module "auth_ldap_group" {
   depends_on = [module.auth_ldap_backend]
 }
 
+module "auth_oidc_backend" {
+  source = "./modules/auth_oidc_backend"
+
+  for_each = var.auth_oidc_backend
+
+  path                = each.key
+  description         = each.value.description
+  oidc_discovery_url  = each.value.oidc_discovery_url
+  client_secret_mount = each.value.client_secret_mount
+  client_secret_path  = each.value.client_secret_path
+  default_role        = each.value.default_role
+  listing_visibility  = each.value.listing_visibility
+  default_lease_ttl   = each.value.default_lease_ttl
+  max_lease_ttl       = each.value.max_lease_ttl
+}
+
+module "auth_oidc_role" {
+  source = "./modules/auth_oidc_role"
+
+  for_each = var.auth_oidc_role
+
+  backend               = each.value.backend
+  role_name             = each.value.role_name
+  user_claim            = each.value.user_claim
+  groups_claim          = each.value.groups_claim
+  oidc_scopes           = each.value.oidc_scopes
+  bound_audiences       = each.value.bound_audiences
+  allowed_redirect_uris = each.value.allowed_redirect_uris
+  token_ttl             = each.value.token_ttl
+  token_max_ttl         = each.value.token_max_ttl
+  # Human authorization comes from the external identity groups below, so a
+  # login role carrying no policies is the normal case.
+  token_policies = try(var.policy_auth_map[each.value.backend][each.value.role_name], [])
+
+  depends_on = [module.auth_oidc_backend]
+}
+
+module "auth_oidc_group" {
+  source = "./modules/auth_oidc_group"
+
+  for_each = var.auth_oidc_group
+
+  groupname      = each.value.groupname
+  policies       = var.policy_auth_map[each.value.backend][each.value.groupname]
+  mount_accessor = module.auth_oidc_backend[each.value.backend].accessor
+
+  depends_on = [module.auth_oidc_backend]
+}
+
 module "auth_kubernetes_backend" {
   source = "./modules/auth_kubernetes_backend"