_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>