feat: add partial shipment support (Phase 3)

- Add shipped_quantity field to OrderItem for tracking partial fulfillment
- Add partially_shipped order status for orders with partial shipments
- Add fulfill_item method for shipping individual items with quantities
- Add get_shipment_status method for detailed shipment tracking
- Add vendor API endpoints for partial shipment operations:
  - GET /orders/{id}/shipment-status - Get item-level shipment status
  - POST /orders/{id}/items/{item_id}/ship - Ship specific item quantity
- Automatic status updates: partially_shipped when some items shipped,
  shipped when all items fully shipped
- Migration to add shipped_quantity column with upgrade for existing data
- Update documentation with partial shipment usage examples

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/services/order_service.py

@@ -925,6 +925,7 @@ class OrderService:
         stats = {
             "pending": 0,
             "processing": 0,
+            "partially_shipped": 0,
             "shipped": 0,
             "delivered": 0,
             "cancelled": 0,
@@ -989,6 +990,9 @@ class OrderService:
             # Update timestamps based on status
             if order_update.status == "processing" and not order.confirmed_at:
                 order.confirmed_at = now
+            elif order_update.status == "partially_shipped":
+                # partially_shipped doesn't set shipped_at yet
+                pass
             elif order_update.status == "shipped" and not order.shipped_at:
                 order.shipped_at = now
             elif order_update.status == "delivered" and not order.delivered_at:
@@ -1256,6 +1260,7 @@ class OrderService:
             "total_orders": 0,
             "pending_orders": 0,
             "processing_orders": 0,
+            "partially_shipped_orders": 0,
             "shipped_orders": 0,
             "delivered_orders": 0,
             "cancelled_orders": 0,