fix: add customer_id parameter to get_vendor_orders service method

The vendor orders API endpoint was passing customer_id to the service
but the service method didn't accept it, causing a TypeError.

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

app/services/order_service.py

@@ -858,6 +858,7 @@ class OrderService:
         status: str | None = None,
         channel: str | None = None,
         search: str | None = None,
+        customer_id: int | None = None,
     ) -> tuple[list[Order], int]:
         """
         Get orders for vendor with filtering.
@@ -870,6 +871,7 @@ class OrderService:
             status: Filter by status
             channel: Filter by channel (direct, letzshop)
             search: Search by order number, customer name, or email
+            customer_id: Filter by customer ID
 
         Returns:
             Tuple of (orders, total_count)
@@ -882,6 +884,9 @@ class OrderService:
         if channel:
             query = query.filter(Order.channel == channel)
 
+        if customer_id:
+            query = query.filter(Order.customer_id == customer_id)
+
         if search:
             search_term = f"%{search}%"
             query = query.filter(