Changes

add shopify full product sync button to order source form

Visible only for shopify-new order sources: queues a background full
sync (all articles) in the Laravel middleware via new server route
/api/settings/order-sources/[id]/sync-shopify-products. The middleware
holds a per-source lock, so the manual sync waits instead of running
concurrently with the scheduled one.

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