feat: Add sorting by exclusive labels (issue priority) (#33206)

Fix #2616

This PR adds a new sort option for exclusive labels.

For exclusive labels, a new property is exposed called "order", while in
the UI options are populated automatically in the `Sort` column (see
screenshot below) for each exclusive label scope.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/indexer/issues/db/options.go

@@ -6,6 +6,7 @@ package db
 import (
 	"context"
 	"fmt"
+	"strings"
 
 	"code.gitea.io/gitea/models/db"
 	issue_model "code.gitea.io/gitea/models/issues"
@@ -34,7 +35,11 @@ func ToDBOptions(ctx context.Context, options *internal.SearchOptions) (*issue_m
 	case internal.SortByDeadlineAsc:
 		sortType = "nearduedate"
 	default:
-		sortType = "newest"
+		if strings.HasPrefix(string(options.SortBy), issue_model.ScopeSortPrefix) {
+			sortType = string(options.SortBy)
+		} else {
+			sortType = "newest"
+		}
 	}
 
 	// See the comment of issues_model.SearchOptions for the reason why we need to convert
@@ -68,7 +73,6 @@ func ToDBOptions(ctx context.Context, options *internal.SearchOptions) (*issue_m
 		ExcludedLabelNames: nil,
 		IncludeMilestones:  nil,
 		SortType:           sortType,
-		IssueIDs:           nil,
 		UpdatedAfterUnix:   options.UpdatedAfterUnix.Value(),
 		UpdatedBeforeUnix:  options.UpdatedBeforeUnix.Value(),
 		PriorityRepoID:     0,