WP-12: contract and failing tests for the Kubernetes overview

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

backend/crates/application/src/lib.rs

@@ -1,5 +1,6 @@
 //! Application layer: use cases orchestrating the domain through its ports.
 pub mod auth_service;
+pub mod cluster_service;
 pub mod inventory_service;
 pub mod jobs;
 pub mod scheduler;
@@ -8,6 +9,7 @@ pub mod upgrade_service;
 pub mod user_service;
 
 pub use auth_service::AuthService;
+pub use cluster_service::ClusterService;
 pub use inventory_service::{InventoryService, PackageRefreshJob};
 pub use jobs::{JobHandler, JobLog, JobRunner};
 pub use settings_service::SettingsService;