fix: resolve kustomize namespace conflict for gooseek-sandbox

- Extracted gooseek-sandbox Namespace to separate file
- Apply sandbox-namespace.yaml separately before kustomize
- Fixed deprecated commonLabels -> labels
- Prevents namespace transformation conflict

Made-with: Cursor

backend/deploy/k8s/kustomization.yaml

@@ -26,9 +26,11 @@ resources:
   - opensandbox.yaml
   - ingress.yaml
 
-commonLabels:
-  app.kubernetes.io/part-of: gooseek
-  app.kubernetes.io/managed-by: kustomize
+labels:
+  - pairs:
+      app.kubernetes.io/part-of: gooseek
+      app.kubernetes.io/managed-by: kustomize
+    includeSelectors: false
 
 images:
   - name: gooseek/backend