fix(api): address review feedback on project board API

Three issues raised by @lunny in review of #36008 are addressed:

1. Duplicate permission checks removed
   The /projects route group is already wrapped with reqRepoReader and
   reqRepoWriter in api.go. The inline CanRead/CanWrite checks at the
   top of all 10 handlers were unreachable dead code.

2. AddOrUpdateIssueToColumn replaced with IssueAssignOrRemoveProject
   The custom function introduced in #36008 was missing a db.WithTx
   transaction wrapper, the CommentTypeProject audit comment written by
   the UI, and the CanBeAccessedByOwnerRepo cross-repo ownership guard.
   AddIssueToProjectColumn now delegates to the existing
   IssueAssignOrRemoveProject which provides all three.

3. ListProjectColumns pagination implemented correctly
   Added CountColumns and GetColumnsPaginated (using
   db.SetSessionPagination) to the project model. The handler uses
   utils.GetListOptions and sets X-Total-Count via
   ctx.SetTotalCountHeader per API contribution guidelines.
   Integration tests cover full list, page 1, page 2, and 404.

Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

routers/api/v1/repo/project.go

@@ -13,10 +13,10 @@ import (
 	"code.gitea.io/gitea/modules/setting"
 	api "code.gitea.io/gitea/modules/structs"
 	"code.gitea.io/gitea/modules/web"
+	"code.gitea.io/gitea/routers/api/v1/utils"
 	"code.gitea.io/gitea/services/context"
 	"code.gitea.io/gitea/services/convert"
 	project_service "code.gitea.io/gitea/services/projects"
-	"code.gitea.io/gitea/routers/api/v1/utils"
 )
 
 // ListProjects lists all projects in a repository
@@ -398,6 +398,7 @@ func ListProjectColumns(ctx *context.APIContext) {
 		return
 	}
 
+	ctx.SetLinkHeader(int(total), listOptions.PageSize)
 	ctx.SetTotalCountHeader(total)
 	ctx.JSON(http.StatusOK, convert.ToProjectColumnList(ctx, columns))
 }