Add Vault OIDC authentication via Authentik

Configure Vault to use Authentik as OIDC identity provider for SSO login.
Creates OAuth2 provider/application in Authentik, adds OIDC auth backend,
admin policy, and maps "authentik Admins" group to full vault-admin access.

stacks/vault/terragrunt.hcl

@@ -0,0 +1,8 @@
+include "root" {
+  path = find_in_parent_folders()
+}
+
+dependency "platform" {
+  config_path  = "../platform"
+  skip_outputs = true
+}