shopper/framework: Race condition on Discount.usage_limit allows silent over-redemption
CreateOrderFromCartAction::execute previously created the Order row before checking and incrementing the discount's total_use counter. Under concurrent checkout pressure (Black Friday, flash sale, viral coupon), the global usage_limit was silently exceeded: orders were committed with the discount fully applied to price_amount while the counter blocked at usage_limit. The merchant had no signal that an over-redemption had occurred. A second related bug: usage_limit_per_user was effectively a no-op because the counter it relied …