fix CI deploy: align image var name (BE_IMAGE → IMAGE)

k8s/backend.yaml referenced ${BE_IMAGE} but the workflow env
defines ${IMAGE}, causing envsubst to produce an empty image
name (InvalidImageName). Also consolidated the summary table
to single image line since FE/BE are merged.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

k8s/backend.yaml

@@ -54,7 +54,7 @@ spec:
     spec:
       containers:
       - name: backend
-        image: ${BE_IMAGE}:${IMAGE_TAG}
+        image: ${IMAGE}:${IMAGE_TAG}
         imagePullPolicy: Always
         ports:
         - containerPort: 8080