Fulfillments: Assign A Supplier (Dropship)

A fulfillment can now carry an assigned supplier. The fulfillment detail page has a new Supplier card to assign, change, or remove the supplier; fulfillments with a supplier present show a Dropship badge, and the fulfillment orders list gains supplier and Dropship columns and filters. The underlying record stays neutral — it's an association to a supplier — so the same data can later be reported on by supplier without a redesign; the Dropship treatment lives in the UI.