fix(09-04): keep relation columns out of the default sort set

- A relation column is not sortable unless columns.yaml says so
- An explicit sortable relation orders the joined select column, then the primary key

cabana/list_schema.go

@@ -227,7 +227,7 @@ func compileColumns(pluginID string, ctl pact.AdminController, fsys fs.FS, colPa
 				return nil, bootErr(pluginID, ctl.ID(), colPath, fmt.Errorf("column %s is not on the model", key))
 			}
 		}
-		sortable := true
+		sortable := spec.Relation == ""
 		if spec.Sortable != nil {
 			sortable = *spec.Sortable
 		}

cabana/query.go

@@ -123,7 +123,7 @@ func ExecuteList(ctx context.Context, db *gorm.DB, cc *CompiledController, in Li
 	if err := q.Session(&gorm.Session{}).Count(&total).Error; err != nil {
 		return nil, err
 	}
-	q = applyListOrder(q, model, sortCol, sortDesc)
+	q = applyListOrder(q, cc.List, model, sortCol, sortDesc)
 	for _, col := range cc.List.Columns {
 		if col.Relation != "" {
 			q = q.Preload(col.Relation)
@@ -207,7 +207,7 @@ func normalizeSort(schema *ListSchema, in ListInput) (string, bool, error) {
 			break
 		}
 	}
-	if match == nil || !match.Sortable || match.Relation != "" {
+	if match == nil || !match.Sortable {
 		return "", false, listInvalid("sort", "is not a sortable column")
 	}
 	dir := in.Dir
@@ -220,12 +220,23 @@ func normalizeSort(schema *ListSchema, in ListInput) (string, bool, error) {
 	return match.Key, dir == "desc", nil
 }
 
-func applyListOrder(db *gorm.DB, model any, column string, desc bool) *gorm.DB {
+func applyListOrder(db *gorm.DB, schema *ListSchema, model any, column string, desc bool) *gorm.DB {
 	table := tableName(model)
 	pk := primaryColumn(model)
 	if column != "" {
+		orderTable := table
+		orderColumn := column
+		for _, col := range schema.Columns {
+			if col.Key != column || col.Relation == "" {
+				continue
+			}
+			orderTable = col.Relation
+			orderColumn = col.Select
+			db = db.Joins(col.Relation)
+			break
+		}
 		db = db.Order(clause.OrderByColumn{
-			Column: clause.Column{Table: table, Name: column},
+			Column: clause.Column{Table: orderTable, Name: orderColumn},
 			Desc:   desc,
 		})
 	}