feat: notification sending for printing order created

printing/signals.py

@@ -0,0 +1,17 @@
+"""
+Printing domain signals.
+
+We use domain-level signals (instead of model post_save) to:
+- keep semantics clean (only fire on business "create" flow)
+- allow passing explicit actor (created_by / sender_label)
+- keep integration (e.g. WeCom notify) in handlers
+"""
+
+from django.dispatch import Signal
+
+# Fired when a PrintingOrder is created via business service.
+# Payload:
+# - instance: PrintingOrder
+# - created_by: Django User (may be None)
+printing_order_created = Signal()
+