Add preview url to json

src/request.ts

@@ -64,6 +64,7 @@ export default class ScreenshotRequest {
     elapsed_time_s: number;
     zip_url: string | null;
     current_action: string;
+    preview: string;
   }> {
     return {
       url: this.url,
@@ -89,6 +90,7 @@ export default class ScreenshotRequest {
         this.status === "finished"
           ? `/api/requests/${this.id}/all-screenshots`
           : null,
+      preview: `/static/${this.id}/preview.png`,
     };
   }