Completion feed

Pass completed_since (an ISO-8601 instant) to pull every order completed recently — the typical ERP write-back trigger:

This returns only orders with completed_at >= completed_since, ordered by completed_at ascending, and paginates with the same opaque cursor (keep completed_since present on every page). Because completed_at is refreshed on every completion, a re-exported order reappears — pair it with the order.completed webhook for a robust push-plus-reconcile integration.