Allow multiple projects per issue and pull requests (#36784)

Add ability to add and remove multiple projects per issue
and pull request.

Resolve #12974

---------

Signed-off-by: Icy Avocado <avocado@ovacoda.com>
Co-authored-by: Tyrone Yeh <siryeh@gmail.com>
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: OpenCode (gpt-5.2-codex) <opencode@openai.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>

routers/web/repo/pull.go

@@ -1301,7 +1301,7 @@ func CompareAndPullRequestPost(ctx *context.Context) {
 		return
 	}
 
-	labelIDs, assigneeIDs, milestoneID, projectID := validateRet.LabelIDs, validateRet.AssigneeIDs, validateRet.MilestoneID, validateRet.ProjectID
+	labelIDs, assigneeIDs, milestoneID, projectIDs := validateRet.LabelIDs, validateRet.AssigneeIDs, validateRet.MilestoneID, validateRet.ProjectIDs
 
 	var attachments []string
 	if setting.Attachment.Enabled {
@@ -1368,7 +1368,7 @@ func CompareAndPullRequestPost(ctx *context.Context) {
 		AssigneeIDs:     assigneeIDs,
 		Reviewers:       validateRet.Reviewers,
 		TeamReviewers:   validateRet.TeamReviewers,
-		ProjectID:       projectID,
+		ProjectIDs:      projectIDs,
 	}
 	if err := pull_service.NewPullRequest(ctx, prOpts); err != nil {
 		switch {