@Transactional
public PaymentUrlResponseDto preparePayment(Long userId, Integer amount, String orderName) {
    User user = userRepository.findByIdOrElseThrow(userId);
    
    // 고유한 주문 ID 생성 (UUID + timestamp)
    String orderId = generateOrderId();
    
    // 결제 정보 저장 (PENDING 상태)
    Payment payment = Payment.builder()
        .orderId(orderId)
        .orderName(orderName)
        .amount(amount)
        .status(PaymentStatus.PENDING)
        .user(user)
        .build();
    
    paymentRepository.save(payment);
    
    return new PaymentUrlResponseDto(
        orderId, amount, orderName, user.getEmail(), user.getName(),
        tossPaymentConfig.getSuccessUrl(), tossPaymentConfig.getFailUrl()
    );
}

@Transactional
public PaymentResponseDto confirmPayment(String paymentKey, String orderId, Integer amount) {
    // 1. 주문 정보 조회
    Payment payment = paymentRepository.findByOrderIdOrElseThrow(orderId);
    
    // 2. 결제 금액 검증
    if (!payment.getAmount().equals(amount)) {
        throw new BaseException(ExceptionCode.PAYMENT_AMOUNT_MISMATCH);
    }
    
    try {
        // 3. 토스페이먼츠 결제 승인 요청
        PaymentResponseDto tossResponse = tossPaymentClient.confirmPayment(
            paymentKey, orderId, amount);
        
        // 4. 결제 정보 업데이트
        payment.updatePaymentKey(paymentKey);
        payment.updateStatus(PaymentStatus.CONFIRMED);
        payment.updateMethod(tossResponse.method());
        
        // 5. 포인트 충전
        pointService.chargePoint(
            payment.getUser().getId(), 
            payment.getAmount(), 
            "토스페이먼츠 충전 - " + payment.getOrderName()
        );
        
        return tossResponse;
        
    } catch (Exception e) {
        payment.updateStatus(PaymentStatus.FAILED);
        payment.updateFailureReason(e.getMessage());
        throw new BaseException(ExceptionCode.TOSS_PAYMENT_CONFIRM_FAILED);
    }
}

@Component
public class TossPaymentClient {
    
    // 결제 승인
    public PaymentResponseDto confirmPayment(String paymentKey, String orderId, Integer amount);
    
    // 결제 조회
    public PaymentResponseDto getPayment(String paymentKey);
    
    // 결제 취소
    public PaymentResponseDto cancelPayment(String paymentKey, String cancelReason);
}

private String encodeSecretKey(String secretKey) {
    return Base64.getEncoder()
        .encodeToString((secretKey + ":").getBytes(StandardCharsets.UTF_8));
}

HttpHeaders headers = new HttpHeaders();
headers.set(HttpHeaders.AUTHORIZATION, "Basic " + auth);
headers.set(HttpHeaders.CONTENT_TYPE, MediaType.APPLICATION_JSON_VALUE);

@Transactional
public void failPayment(String orderId, String failureReason) {
    Payment payment = paymentRepository.findByOrderIdOrElseThrow(orderId);
    
    payment.updateStatus(PaymentStatus.FAILED);
    payment.updateFailureReason(failureReason);
    
    log.info("결제 실패 처리 완료 - orderId: {}, reason: {}", orderId, failureReason);
}

@Transactional
public PaymentResponseDto cancelPayment(String orderId, String cancelReason) {
    // 1. 주문 정보 조회
    Payment payment = paymentRepository.findByOrderIdOrElseThrow(orderId);
    
    // 2. 취소 가능한 상태인지 확인
    if (payment.getStatus() != PaymentStatus.CONFIRMED) {
        throw new BaseException(ExceptionCode.PAYMENT_NOT_CANCELLABLE);
    }
    
    try {
        // 3. 토스페이먼츠 결제 취소 요청
        PaymentResponseDto tossResponse = tossPaymentClient.cancelPayment(
            payment.getPaymentKey(), cancelReason);
        
        // 4. 결제 정보 업데이트
        payment.updateStatus(PaymentStatus.CANCELLED);
        payment.updateFailureReason(cancelReason);
        
        // 5. 포인트 차감 (충전했던 포인트 되돌리기)
        pointService.usePoint(
            payment.getUser().getId(), 
            payment.getAmount(), 
            "결제 취소로 인한 포인트 차감 - " + payment.getOrderName()
        );
        
        return tossResponse;
        
    } catch (Exception e) {
        throw new BaseException(ExceptionCode.TOSS_PAYMENT_CANCEL_FAILED);
    }
}

@Entity
public class Payment extends BaseEntity {
    private String orderId;        // 주문 ID (유니크)
    private String paymentKey;     // 토스 결제 키
    private String orderName;      // 주문명
    private Integer amount;        // 결제 금액
    private PaymentStatus status;  // 결제 상태
    private String method;         // 결제 수단
    private User user;             // 결제 사용자
    private String failureReason;  // 실패 사유
}

@Transactional(readOnly = true)
public List<Payment> getPaymentHistory(Long userId) {
    return paymentRepository.findByUserIdOrderByCreatedAtDesc(userId);
}

// 결제 금액 검증
if (!payment.getAmount().equals(amount)) {
    throw new BaseException(ExceptionCode.PAYMENT_AMOUNT_MISMATCH);
}

// 결제 상태 검증
if (payment.getStatus() != PaymentStatus.PENDING) {
    throw new BaseException(ExceptionCode.INVALID_PAYMENT_STATUS);
}