Load mentionValues asynchronously (#36739)

Eliminate a few database queries on all issue and pull request pages by
moving mention autocomplete data to async JSON endpoints fetched
on-demand when the user types `@`.

See https://github.com/go-gitea/gitea/pull/36739#issuecomment-3963184858
for the full table of affected pages.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

routers/web/repo/issue_list.go

@@ -662,10 +662,7 @@ func prepareIssueFilterAndList(ctx *context.Context, milestoneID, projectID int6
 		ctx.ServerError("GetRepoAssignees", err)
 		return
 	}
-	handleMentionableAssigneesAndTeams(ctx, shared_user.MakeSelfOnTop(ctx.Doer, assigneeUsers))
-	if ctx.Written() {
-		return
-	}
+	ctx.Data["Assignees"] = shared_user.MakeSelfOnTop(ctx.Doer, assigneeUsers)
 
 	ctx.Data["IssueRefEndNames"], ctx.Data["IssueRefURLs"] = issue_service.GetRefEndNamesAndURLs(issues, ctx.Repo.RepoLink)