fix: use manual res.json() in getNextJob to prevent double-response on 204

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

paperless-backend/src/label-print-agent/label-print-agent.controller.ts

@@ -60,19 +60,19 @@ export class LabelPrintAgentController {
 
   // Agent: nächsten Job abholen (Polling)
   @Get('jobs/next')
-  async getNextJob(@Query('agentId') agentId: string, @Res({ passthrough: true }) res: Response) {
+  async getNextJob(@Query('agentId') agentId: string, @Res() res: Response) {
     const job = await this.service.claimNextJob(agentId ?? 'unknown');
     if (!job) {
       res.status(HttpStatus.NO_CONTENT).send();
       return;
     }
-    return {
+    res.status(HttpStatus.OK).json({
       jobId: String(job.Id),
       labelImageBase64: job.LabelImageData ? job.LabelImageData.toString('base64') : null,
       labelImageContentType: 'image/png',
       labelWidthMm: job.LabelWidthMm,
       labelHeightMm: job.LabelHeightMm,
-    };
+    });
   }
 
   // Agent: Bild separat abrufen