Proposal: fulfillment-invoices

Why

After payment, merchants ship goods (发货单) and customers request invoices (发票). This phase closes the post-order loop of the MVP.

Schema: shipments (carrier, tracking, status, per-item qty), invoices (title, tax_no, kind, amount, status).
Shipment lifecycle: created(pending) → shipped → delivered; order becomes fulfilling on first shipment, shipped when all items shipped, completed when all delivered (customer confirm allowed).
Merchant APIs: create shipment for paid/fulfilling orders, mark shipped, list shipments/invoices, issue invoice.
Customer APIs: my shipments, confirm delivery, request invoice per order, my invoices.