fix(e2e): correct Service selector to match Deployment labels

The Service selector was using app: "${test_name}-backend" but the
Deployment pod template has app: backend. Fixed selector to match
the actual pod labels so endpoints are created correctly.

Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: Andrei Kvapil <kvapss@gmail.com>
(cherry picked from commit 3a5977ff60)

hack/e2e-apps/run-kubernetes.sh

@@ -174,7 +174,8 @@ metadata:
 spec:
   type: LoadBalancer
   selector:
-    app: "${test_name}-backend"
+    app: backend
+    backend: "${test_name}-backend"
   ports:
   - port: 80
     targetPort: 80