Global exception handling with @ControllerAdvice and standard error contract

[sugan0tech]

• Description: Centralize error handling and return a consistent error payload.
• Current state: Controllers build ad-hoc Map responses; no @ControllerAdvice present.
• Tasks:
  • Add spring-boot-starter-validation if needed for error binding; create GlobalExceptionHandler with handlers for MethodArgumentNotValidException, AccessDeniedException, EntityNotFoundException, and fallback.
  • Define standard error JSON: {timestamp, status, error, message, path, traceId}.
  • Replace manual map returns as appropriate.
• Acceptance criteria:
  • Errors across endpoints follow the contract; tests assert shape.
• References: Controllers under features/*.