feat: 实现注册优惠码功能

- 支持创建/编辑/删除优惠码，设置赠送金额和使用限制
  - 注册页面实时验证优惠码并显示赠送金额
  - 支持 URL 参数自动填充 (?promo=CODE)
  - 添加优惠码验证接口速率限制
  - 使用数据库行锁防止并发超限
  - 新增后台优惠码管理页面，支持复制注册链接

backend/internal/service/auth_service.go

@@ -52,6 +52,7 @@ type AuthService struct {
 	emailService      *EmailService
 	turnstileService  *TurnstileService
 	emailQueueService *EmailQueueService
+	promoService      *PromoService
 }
 
 // NewAuthService 创建认证服务实例
@@ -62,6 +63,7 @@ func NewAuthService(
 	emailService *EmailService,
 	turnstileService *TurnstileService,
 	emailQueueService *EmailQueueService,
+	promoService *PromoService,
 ) *AuthService {
 	return &AuthService{
 		userRepo:          userRepo,
@@ -70,16 +72,17 @@ func NewAuthService(
 		emailService:      emailService,
 		turnstileService:  turnstileService,
 		emailQueueService: emailQueueService,
+		promoService:      promoService,
 	}
 }
 
 // Register 用户注册，返回token和用户
 func (s *AuthService) Register(ctx context.Context, email, password string) (string, *User, error) {
-	return s.RegisterWithVerification(ctx, email, password, "")
+	return s.RegisterWithVerification(ctx, email, password, "", "")
 }
 
-// RegisterWithVerification 用户注册（支持邮件验证），返回token和用户
-func (s *AuthService) RegisterWithVerification(ctx context.Context, email, password, verifyCode string) (string, *User, error) {
+// RegisterWithVerification 用户注册（支持邮件验证和优惠码），返回token和用户
+func (s *AuthService) RegisterWithVerification(ctx context.Context, email, password, verifyCode, promoCode string) (string, *User, error) {
 	// 检查是否开放注册（默认关闭：settingService 未配置时不允许注册）
 	if s.settingService == nil || !s.settingService.IsRegistrationEnabled(ctx) {
 		return "", nil, ErrRegDisabled
@@ -150,6 +153,19 @@ func (s *AuthService) RegisterWithVerification(ctx context.Context, email, passw
 		return "", nil, ErrServiceUnavailable
 	}
 
+	// 应用优惠码（如果提供）
+	if promoCode != "" && s.promoService != nil {
+		if err := s.promoService.ApplyPromoCode(ctx, user.ID, promoCode); err != nil {
+			// 优惠码应用失败不影响注册，只记录日志
+			log.Printf("[Auth] Failed to apply promo code for user %d: %v", user.ID, err)
+		} else {
+			// 重新获取用户信息以获取更新后的余额
+			if updatedUser, err := s.userRepo.GetByID(ctx, user.ID); err == nil {
+				user = updatedUser
+			}
+		}
+	}
+
 	// 生成token
 	token, err := s.GenerateToken(user)
 	if err != nil {