Make item upload/update configurable and move this task to background

- each item should have a checkbox to control whether that item needs to uploaded/updated to Shopify.
- Item update/upload should happen via background job and not a hook.

Trace the existing item upload/update hook and the item data model or configuration it uses. Identify how the project's background jobs are started, then define the checkbox behavior and move Shopify upload/update processing out of the hook. Done means each item can control syncing and the work runs through a background job.

Written by the indexing model from the issue text.