layered-architecturelisted

## Purpose Prevent architectural drift in Spring Boot applications by enforcing clear layer boundaries. ## Checklist - **Controller layer**: handles HTTP concerns only (request mapping, validation, response serialization). Never contains business logic or data access. - **Service layer**: contains business logic and transaction boundaries. Never returns JPA entities to controllers — use DTOs or projections. - **Repository layer**: data access only. Spring Data JPA repositories or custom DAO implementations. Never contains business logic. - **DTO boundaries**: controllers receive request DTOs and return response DTOs. Services receive and return domain objects or DTOs. Entities never cross the controller boundary. - **@Transactional**: on service-layer methods, not controllers. Read-only transactions for query methods. - **Exception hierarchy**: domain exceptions (extending `RuntimeException`) caught at the controller boundary and mapped to appropriate HTTP status codes via `@ControllerAdvice`. ## Expected output A Spring Boot application where each layer has a single responsibility, entities never reach controllers, and exceptions are consistently mapped to HTTP responses.