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/package.json

@@ -29,6 +29,7 @@
     "@nestjs/jwt": "^11.0.2",
     "@nestjs/passport": "^11.0.5",
     "@nestjs/platform-express": "^11.0.1",
+    "@nestjs/schedule": "^12.0.1",
     "@nestjs/swagger": "^11.4.7",
     "@nestjs/throttler": "^6.5.0",
     "@prisma/adapter-pg": "^7.9.1",