fix: allow empty extra payload to clear account quota limits

UpdateAccount previously required len(input.Extra) > 0, causing explicit
empty payloads (extra:{}) to be silently skipped. Change condition to
input.Extra != nil so clearing quota keys actually persists.

backend/internal/service/admin_service.go

@@ -1530,7 +1530,9 @@ func (s *adminServiceImpl) UpdateAccount(ctx context.Context, id int64, input *U
 	if len(input.Credentials) > 0 {
 		account.Credentials = input.Credentials
 	}
-	if len(input.Extra) > 0 {
+	// Extra 使用 map：需要区分“未提供(nil)”与“显式清空({})”。
+	// 关闭配额限制时前端会删除 quota_* 键并提交 extra:{}，此时也必须落库。
+	if input.Extra != nil {
 		// 保留配额用量字段，防止编辑账号时意外重置
 		for _, key := range []string{"quota_used", "quota_daily_used", "quota_daily_start", "quota_weekly_used", "quota_weekly_start"} {
 			if v, ok := account.Extra[key]; ok {