feat: field pre_purchase_order_id for purchase_order (allow null)

business/models.py

@@ -130,6 +130,11 @@ class PurchaseOrder(OrderItemsAggregationMixin, OrderDirectionMixin, OrderCounte
         default=PurchaseOrderStatusEnum.PENDING, 
         verbose_name='状态',
     )
+    from_pre_purchase_order_id = models.BigIntegerField(
+        null=True,
+        blank=True,
+        verbose_name='关联预采购单ID',
+    )
     remarks = models.TextField(blank=True, null=True, verbose_name='备注')
 
     def __str__(self):