Use per-deploy preview hosts under the site root domain.

Build hosts as <userPrefix>-<deploymentNumber>-preview.<rootDomain> from FRONTEND_URL, wire them through ingress/TLS, and open them from the preview API.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/src/applications/applications.controller.ts

@@ -409,7 +409,10 @@ export class ApplicationsController {
   @ApiOperation({ summary: 'Get preview URL for the deployed application' })
   async getPreview(@Param('id') id: string, @Request() req: any) {
     const app = await this.applicationsService.findOne(id, this.staffUserIdFilter(req));
-    return this.kubernetesService.getPreviewInfo(app);
+    const deployments = await this.deploymentsService.findByApplication(app.id);
+    const latest = deployments.find((d) => d.status === 'running') || deployments[0];
+    const previewNumber = latest?.previewSubdomain ?? null;
+    return this.kubernetesService.getPreviewInfo(app, previewNumber);
   }
 
   @Post(':id/access')