private async Task <(string, DateTime, bool)> GenerateRss(string group)
{
var contents = await _contentsService.All()
.Where(c => string.Equals(c.Group, group, StringComparison.OrdinalIgnoreCase))
.OrderByDescending(c => c.Created).Take(MaxRssItemCount).ToListAsync();
if (contents.Count == 0)
{
return(null, default, true);