fix: add explicit type annotations to service methods

Add Optional type annotations to nullable parameters in service methods
to fix architecture validation warnings.

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

app/services/vendor_service.py

@@ -453,7 +453,7 @@ class VendorService:
                     vendor.vendor_code, product.marketplace_product_id
                 )
 
-            # Create vendor -product association
+            # Create vendor-product association
             new_product = Product(
                 vendor_id=vendor.id,
                 marketplace_product_id=marketplace_product.id,