عرض عملي: جعل عملية الدفع (Checkout) ذرّية

خطوة بخطوة

1. Show placeOrder without @Transactional: stock is reserved, then the order save fails; stock is wrong.
2. Add @Transactional to the service method.
3. Re-run the failing test: stock is unchanged.
4. Planned mistake: throw a checked exception and show the stock change committed; fix with rollbackFor.
5. Add @Transactional(readOnly = true) to the order query method.