fix: added merchant_id to printing_order and plate_order

shipment/models.py

@@ -1,8 +1,14 @@
+from __future__ import annotations
+from typing import TYPE_CHECKING
+
 from django.db import models
 from django.contrib.auth import get_user_model
 from flower.common import ModelBase
 from basic_info import models as basic_models
 
+if TYPE_CHECKING:
+    from printing.models import PrintingJob
+
 User = get_user_model()
 
 
@@ -138,7 +144,7 @@ class SalesItem(ModelBase):
     def __str__(self):
         return f'{self.name} x {self.quantity} {self.get_unit_display()}'
     
-    def get_printing_job(self):
+    def get_printing_job(self) -> PrintingJob | None:
         """
         获取关联的 PrintingJob 实例
         
@@ -150,10 +156,10 @@ class SalesItem(ModelBase):
         try:
             from printing.models import PrintingJob
             return PrintingJob.objects.get(id=self.printing_job_id)
-        except Exception:
+        except PrintingJob.DoesNotExist:
             return None
     
-    def get_customer(self):
+    def get_customer(self) -> basic_models.Customer | None:
         """
         获取关联的 Customer 实例