fix: round 3 audit fixes - SMTP header sanitization and goroutine safety

- Move sanitizeEmailHeader to SendEmailWithConfig entry point, covering all
  email senders (verify code, password reset, ops alerts, notifications)
- Add panic recovery to UpdateBalance goroutine
- Fix stale comment in getAccountQuotaNotifyEmails (email="" no longer used)
- Log error instead of silently discarding verifyNotifyCode cache update failure

backend/cmd/server/VERSION

@@ -1 +1 @@
-0.1.110.20
+0.1.110.21

backend/internal/service/balance_notify_service.go

@@ -225,7 +225,7 @@ func (s *BalanceNotifyService) isAccountQuotaNotifyEnabled(ctx context.Context)
 }
 
 // getAccountQuotaNotifyEmails reads admin notification emails from settings,
-// filtering out disabled entries. Entries with email="" are resolved to the first admin's email.
+// filtering out disabled and unverified entries.
 func (s *BalanceNotifyService) getAccountQuotaNotifyEmails(ctx context.Context) []string {
 	raw, err := s.settingRepo.GetValue(ctx, SettingKeyAccountQuotaNotifyEmails)
 	if err != nil || strings.TrimSpace(raw) == "" || raw == "[]" {

backend/internal/service/email_service.go

@@ -153,9 +153,13 @@ func (s *EmailService) SendEmail(ctx context.Context, to, subject, body string)
 
 // SendEmailWithConfig 使用指定配置发送邮件
 func (s *EmailService) SendEmailWithConfig(config *SMTPConfig, to, subject, body string) error {
-	from := config.From
+	// Sanitize all SMTP header fields to prevent header injection (CR/LF removal).
+	to = sanitizeEmailHeader(to)
+	subject = sanitizeEmailHeader(subject)
+
+	from := sanitizeEmailHeader(config.From)
 	if config.FromName != "" {
-		from = fmt.Sprintf("%s <%s>", config.FromName, config.From)
+		from = fmt.Sprintf("%s <%s>", sanitizeEmailHeader(config.FromName), sanitizeEmailHeader(config.From))
 	}
 
 	msg := fmt.Sprintf("From: %s\r\nTo: %s\r\nSubject: %s\r\nMIME-Version: 1.0\r\nContent-Type: text/html; charset=UTF-8\r\n\r\n%s",

backend/internal/service/user_service.go

@@ -224,6 +224,11 @@ func (s *UserService) UpdateBalance(ctx context.Context, userID int64, amount fl
 	}
 	if s.billingCache != nil {
 		go func() {
+			defer func() {
+				if r := recover(); r != nil {
+					slog.Error("panic in balance cache invalidation", "user_id", userID, "recover", r)
+				}
+			}()
 			cacheCtx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
 			defer cancel()
 			if err := s.billingCache.InvalidateUserBalance(cacheCtx, userID); err != nil {
@@ -359,7 +364,9 @@ func verifyNotifyCode(ctx context.Context, cache EmailCache, email, code string)
 	}
 	if subtle.ConstantTimeCompare([]byte(data.Code), []byte(code)) != 1 {
 		data.Attempts++
-		_ = cache.SetNotifyVerifyCode(ctx, email, data, verifyCodeTTL)
+		if err := cache.SetNotifyVerifyCode(ctx, email, data, verifyCodeTTL); err != nil {
+			slog.Error("failed to update notify verify code attempts", "email", email, "error", err)
+		}
 		if data.Attempts >= maxVerifyCodeAttempts {
 			return ErrVerifyCodeMaxAttempts
 		}