Fix NFS backup destinations broken by the 0.40.0 refactor (0.40.1)

_ensure_helper_image moved to stack_assets_service, but backup_destination_service
imports it lazily inside _nfs_run/_nfs_helper, so nothing failed at import time —
every NFS destination operation raised ImportError at runtime instead. The helper
is now a public ensure_helper_image() and the NFS helpers import it from its new
home. Verified: every services/ and routers/ module imports, and both NFS helper
paths run through to a Docker call instead of ImportError.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "stackpilot-frontend",
   "private": true,
-  "version": "0.40.0",
+  "version": "0.40.1",
   "type": "module",
   "scripts": {
     "dev": "vite",