fix: foreign key constraint error when guest creates order

backend/routers/orders.py

@@ -130,7 +130,7 @@ async def create_order(
         
         # LOG ACTION
         await audit_service.log(
-            user_id=user_id or 0,
+            user_id=user_id,
             action="order_created",
             target_type="order",
             target_id=order_insert_id,

backend/services/audit_service.py

@@ -53,7 +53,7 @@ class AuditService:
 
     async def log(
         self, 
-        user_id: int, 
+        user_id: Optional[int], 
         action: str, 
         target_type: Optional[str] = None, 
         target_id: Optional[int] = None,