fix CI deploy: align k8s resource var names with CI outputs

The k8s/backend.yaml template used BE_-prefixed env vars
(BE_MEMORY_REQUEST, etc.) but the CI workflow outputs them
without the prefix, causing envsubst to produce empty strings
and K8s to reject the deployment.

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

k8s/backend.yaml

@@ -169,11 +169,11 @@ spec:
           mountPath: /data/storage
         resources:
           requests:
-            memory: "${BE_MEMORY_REQUEST}"
-            cpu: "${BE_CPU_REQUEST}"
+            memory: "${MEMORY_REQUEST}"
+            cpu: "${CPU_REQUEST}"
           limits:
-            memory: "${BE_MEMORY_LIMIT}"
-            cpu: "${BE_CPU_LIMIT}"
+            memory: "${MEMORY_LIMIT}"
+            cpu: "${CPU_LIMIT}"
         readinessProbe:
           httpGet:
             path: ${BASE_PATH}/health