use int64_t as id type in shipment model

test/models/shipment.hpp

@@ -17,7 +17,7 @@ struct shipment {
   shipment(const long id, std::string tracking_number)
   : id(id), tracking_number(std::move(tracking_number)) {}
 
-  long id{};
+  int64_t id{};
   std::string tracking_number;
   object::collection<object::object_ptr<package> > packages{};
 
@@ -35,7 +35,7 @@ struct package {
   package(const long id, double weight, object::object_ptr<shipment> del)
   : id(id), weight(weight), delivery(std::move(del)){}
 
-  long id{};
+  int64_t id{};
   double weight{};
   object::object_ptr<shipment> delivery;