feat: implement backend label print agent system for remote label rendering and job management

paperless-backend/src/database/entities/index.ts

@@ -20,3 +20,4 @@ export { InboxDocument } from './inbox-document.entity';
 export { InboxPostprocessingAction } from './inbox-postprocessing-action.entity';
 export { CorrespondentEmailMapping } from './correspondent-email-mapping.entity';
 export { UserSettings } from './user-settings.entity';
+export { LabelPrintJob } from './label-print-job.entity';