Shopper: Race condition on Discount.usage_limit allows silent over-redemption

Shopper is a Headless e-commerce Admin Panel. Prior to 2.8.0, 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. This vulnerability is fixed in 2.8.0.