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

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

backend/crates/application/src/cluster_service.rs

@@ -0,0 +1,37 @@
+use std::sync::Arc;
+
+use domain::cluster::{ClusterOverview, WorkloadRef};
+use domain::ports::ClusterGateway;
+use domain::DomainError;
+
+pub struct ClusterService {
+    gateway: Arc<dyn ClusterGateway>,
+}
+
+impl ClusterService {
+    pub fn new(gateway: Arc<dyn ClusterGateway>) -> Self {
+        let _ = &gateway;
+        Self { gateway }
+    }
+
+    pub async fn overview(&self) -> Result<ClusterOverview, DomainError> {
+        todo!()
+    }
+
+    pub async fn restart(&self, _w: WorkloadRef) -> Result<(), DomainError> {
+        todo!()
+    }
+
+    pub async fn scale(&self, _w: WorkloadRef, _replicas: i32) -> Result<(), DomainError> {
+        todo!()
+    }
+
+    pub async fn set_image(
+        &self,
+        _w: WorkloadRef,
+        _container: Option<String>,
+        _image: &str,
+    ) -> Result<(), DomainError> {
+        todo!()
+    }
+}