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/sandbox-namespace.yaml

@@ -0,0 +1,7 @@
+apiVersion: v1
+kind: Namespace
+metadata:
+  name: gooseek-sandbox
+  labels:
+    app.kubernetes.io/part-of: gooseek
+    purpose: user-sandboxes