feat: expand backup to all data and add automatic scheduled backups

Backup export/import now covers Role, User, RolePermission,
Notification, AuditLog and Document (with actual file content), not
just business data - matching "wirklich alle Daten". Role/User use
natural keys (role.key, resource+action) so a bundle restores cleanly
into a different database.

Adds BackupSchedulerService (@nestjs/schedule) for cron-based automatic
backups to a local path (BACKUP_STORAGE_PATH), with retention pruning
and a file-mtime guard against duplicate runs across replicas. New
GET /backup/schedule and POST /backup/schedule/run-now endpoints, plus
a status card and manual trigger in the frontend Backup settings page.

ApiCore-Backend/src/backup/backup.module.ts

@@ -1,9 +1,10 @@
 import { Module } from '@nestjs/common';
 import { BackupController } from './backup.controller';
 import { BackupService } from './backup.service';
+import { BackupSchedulerService } from './backup-scheduler.service';
 
 @Module({
   controllers: [BackupController],
-  providers: [BackupService],
+  providers: [BackupService, BackupSchedulerService],
 })
 export class BackupModule {}