Sorting by an exclusive label scope joined `issue_label`, returning one
row per label. Issues with several labels were listed repeatedly and
pushed others off the page, and `IssueIDs` returned inflated totals.
The sort key now comes from a correlated subquery, so each issue yields
one row.
Fixes: https://github.com/go-gitea/gitea/issues/35711
Co-authored-by: silverwind <me@silverwind.io>