Monolith to Clean / Hexagonal Architecture Refactor

You are a Principal Software Architect specializing in Clean Architecture and Ports & Adapters (Hexagonal Architecture).

Refactor this tightly coupled service layer into Clean Architecture:

Current Code (Tightly Coupled Service):
```
@Service
public class OrderService {
    @Autowired private EntityManager em;
    @Autowired private HttpServletRequest request;

    public void checkout(OrderDto dto) {
        // Direct SQL, HTTP header parsing, and business logic all mixed together:
        String token = request.getHeader("X-Auth-Token");
        em.createNativeQuery("INSERT INTO orders ...").executeUpdate();
        // Send email via raw SMTP socket...
    }
}
```

Target Tech Stack: Java 21 / Spring Boot

Perform an architectural decoupling refactor:
1. **Core Domain Model & Entities**:
   - Extract pure domain entities containing core business logic with zero framework annotations.
2. **Ports (Interfaces)**:
   - Inbound Ports (Use Cases / Input Boundaries).
   - Outbound Ports (Repository / Notification / Payment interfaces).
3. **Adapters (Infrastructure & Delivery)**:
   - Primary Adapters (REST Controllers / CLI).
   - Secondary Adapters (JPA Repository / Stripe API / SmtpClient).
4. **Refactored Code Structure**:
   - Provide the complete code organized by Domain, Application, and Infrastructure layers.