refactor template builder data

app/controllers/templates_preview_controller.rb

@@ -4,18 +4,7 @@ class TemplatesPreviewController < ApplicationController
   load_and_authorize_resource :template
 
   def show
-    ActiveRecord::Associations::Preloader.new(
-      records: [@template],
-      associations: [{ schema_documents: { preview_images_attachments: :blob } }]
-    ).call
-
-    @template_data =
-      @template.as_json.merge(
-        documents: @template.schema_documents.as_json(
-          methods: %i[metadata signed_key],
-          include: { preview_images: { methods: %i[url metadata filename] } }
-        )
-      ).to_json
+    @template_data = Templates.serialize_for_builder(@template)
 
     render :show, layout: 'plain'
   end