feat: backend MVP (auth/rbac, catalog, orders, fulfillment, invoices) + specs + scaffolds

openspec/changes/fulfillment-invoices/tasks.md

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+# Tasks: fulfillment-invoices
+
+## 1. Schema
+- [ ] Migration: shipments, shipment_items, invoices; status enums; shipment_no/invoice_no sequences
+
+## 2. Shipments
+- [ ] POST /api/shop/orders/{id}/shipments (qty validation vs unshipped remainder)
+- [ ] POST /api/shop/shipments/{id}/ship; order status propagation (fulfilling/shipped)
+- [ ] GET /api/shipments (customer), POST /api/shipments/{id}/confirm-delivered → order completed when done
+- [ ] GET /api/shop/shipments
+
+## 3. Invoices
+- [ ] POST /api/orders/{id}/invoice (one open invoice per order; amount = order total)
+- [ ] GET /api/invoices (customer), GET /api/shop/invoices, POST /api/shop/invoices/{id}/issue
+
+## 4. Tests
+- [ ] cargo test: partial shipment qty math, order status propagation, duplicate invoice rejected, issue flow