feat: add registration abuse prevention

- Silently block verification code for IPs with 2+ registered accounts
- Silently block Gmail alias emails (containing + or . in local part)
- Add CountByRegistrationIP to UserRepository interface
- Pass client IP to SendVerifyCodeAsync for abuse detection

Both checks return fake success to prevent enumeration attacks.

backend/internal/service/auth_service.go

@@ -311,8 +311,9 @@ func (s *AuthService) SendVerifyCode(ctx context.Context, email string) error {
 }
 
 // SendVerifyCodeAsync 异步发送邮箱验证码并返回倒计时
-func (s *AuthService) SendVerifyCodeAsync(ctx context.Context, email string) (*SendVerifyCodeResult, error) {
-	logger.LegacyPrintf("service.auth", "[Auth] SendVerifyCodeAsync called for email: %s", email)
+// clientIP 用于检查同一 IP 注册账号数量限制
+func (s *AuthService) SendVerifyCodeAsync(ctx context.Context, email string, clientIP string) (*SendVerifyCodeResult, error) {
+	logger.LegacyPrintf("service.auth", "[Auth] SendVerifyCodeAsync called for email: %s, ip: %s", email, clientIP)
 
 	// 检查是否开放注册（默认关闭）
 	if s.settingService == nil || !s.settingService.IsRegistrationEnabled(ctx) {
@@ -338,6 +339,28 @@ func (s *AuthService) SendVerifyCodeAsync(ctx context.Context, email string) (*S
 		return nil, ErrEmailExists
 	}
 
+	// 检查 Gmail 别名邮箱（含 + 或本地部分含 . 的），静默假装发送成功
+	if isGmailAliasEmail(email) {
+		logger.LegacyPrintf("service.auth", "[Auth] Gmail alias email detected: %s, returning fake success", email)
+		return &SendVerifyCodeResult{
+			Countdown: 60,
+		}, nil
+	}
+
+	// 检查同一 IP 注册账号数量（>=2 则静默假装发送成功，不实际发送）
+	if clientIP != "" {
+		ipRegCount, err := s.userRepo.CountByRegistrationIP(ctx, clientIP)
+		if err != nil {
+			logger.LegacyPrintf("service.auth", "[Auth] Failed to count users by registration IP: %v", err)
+			// 查询失败不阻塞，继续正常流程
+		} else if ipRegCount >= 2 {
+			logger.LegacyPrintf("service.auth", "[Auth] IP %s already registered %d accounts, returning fake success", clientIP, ipRegCount)
+			return &SendVerifyCodeResult{
+				Countdown: 60,
+			}, nil
+		}
+	}
+
 	// 检查邮件队列服务是否配置
 	if s.emailQueueService == nil {
 		logger.LegacyPrintf("service.auth", "%s", "[Auth] Email queue service not configured")
@@ -1092,6 +1115,22 @@ func isReservedEmail(email string) bool {
 		strings.HasSuffix(normalized, WeChatConnectSyntheticEmailDomain)
 }
 
+// isGmailAliasEmail 检测 Gmail 别名邮箱
+// Gmail 支持两种别名方式：
+// 1. 加号别名：user+anything@gmail.com -> user@gmail.com
+// 2. 点号忽略：u.s.e.r@gmail.com -> user@gmail.com
+// 为防止滥用注册，检测到这类邮箱时返回 true
+func isGmailAliasEmail(email string) bool {
+	normalized := strings.ToLower(strings.TrimSpace(email))
+	if !strings.HasSuffix(normalized, "@gmail.com") {
+		return false
+	}
+	// 提取本地部分（@前面的部分）
+	localPart := strings.TrimSuffix(normalized, "@gmail.com")
+	// 检查是否包含 + 或 .
+	return strings.Contains(localPart, "+") || strings.Contains(localPart, ".")
+}
+
 // GenerateToken 生成JWT access token
 // 使用新的access_token_expire_minutes配置项（如果配置了），否则回退到expire_hour
 func (s *AuthService) GenerateToken(user *User) (string, error) {

backend/internal/service/content_moderation_test.go

@@ -231,6 +231,10 @@ func (r *contentModerationTestUserRepo) DisableTotp(ctx context.Context, userID
 	panic("unexpected DisableTotp call")
 }
 
+func (r *contentModerationTestUserRepo) CountByRegistrationIP(ctx context.Context, ip string) (int, error) {
+	return 0, nil
+}
+
 type contentModerationTestAuthCacheInvalidator struct {
 	userIDs []int64
 }

backend/internal/service/user_service.go

@@ -111,6 +111,9 @@ type UserRepository interface {
 	UpdateTotpSecret(ctx context.Context, userID int64, encryptedSecret *string) error
 	EnableTotp(ctx context.Context, userID int64) error
 	DisableTotp(ctx context.Context, userID int64) error
+
+	// CountByRegistrationIP 统计指定 IP 注册的用户数量
+	CountByRegistrationIP(ctx context.Context, ip string) (int, error)
 }
 
 type UserAuthIdentityRecord struct {