Fix BackupJob import error in backup_cancel function

🐛 Error Fixed:
- NameError: name 'BackupJob' is not defined in backup_cancel view
- Added missing 'from stiftung.models import BackupJob' import

✅ Backup cancellation now works correctly
- Users can cancel long-running backup jobs without errors
- Added local import following the same pattern as other backup functions

The error occurred when trying to cancel a running backup job due to missing model import.

app/stiftung/views.py

@@ -6012,6 +6012,8 @@ def backup_restore(request):
 @login_required
 def backup_cancel(request, backup_id):
     """Cancel a running backup job"""
+    from stiftung.models import BackupJob
+    
     try:
         backup_job = BackupJob.objects.get(id=backup_id)