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/opensandbox.yaml

@@ -1,12 +1,4 @@
 apiVersion: v1
-kind: Namespace
-metadata:
-  name: gooseek-sandbox
-  labels:
-    app.kubernetes.io/part-of: gooseek
-    purpose: user-sandboxes
----
-apiVersion: v1
 kind: ServiceAccount
 metadata:
   name: opensandbox-sa