Fix backup cancellation user reference issues

🐛 Enhanced Error Handling:
- Fixed 'User matching query does not exist' error in backup_cancel
- Handle case where backup_job.created_by is None (deleted user)
- Added error handling for audit logging to prevent cancellation failure
- Improved permission check to handle null created_by references

✅ Backup Cancellation Robustness:
- Now works even if the user who created the backup was deleted
- Audit logging failure won't prevent successful cancellation
- Better error isolation and reporting

The error occurred because the system tried to compare a null user reference
or failed during audit logging when user records were inconsistent.

app/stiftung/views.py

@@ -6023,7 +6023,8 @@ def backup_cancel(request, backup_id):
             return redirect("stiftung:backup_management")
         
         # Check if user has permission to cancel (either own job or admin)
-        if backup_job.created_by != request.user and not request.user.is_staff:
+        # Handle case where created_by might be None (user deleted)
+        if backup_job.created_by is not None and backup_job.created_by != request.user and not request.user.is_staff:
             messages.error(request, "Sie können nur Ihre eigenen Backup-Jobs abbrechen.")
             return redirect("stiftung:backup_management")
         
@@ -6034,14 +6035,18 @@ def backup_cancel(request, backup_id):
         backup_job.error_message = f"Abgebrochen von {request.user.username}"
         backup_job.save()
         
-        # Log the cancellation
-        from stiftung.audit import log_system_action
-        log_system_action(
-            request=request,
-            action="backup_cancel",
-            description=f"Backup-Job abgebrochen: {backup_job.get_backup_type_display()}",
-            details={"backup_job_id": str(backup_job.id)},
-        )
+        # Log the cancellation (with error handling)
+        try:
+            from stiftung.audit import log_system_action
+            log_system_action(
+                request=request,
+                action="backup_cancel",
+                description=f"Backup-Job abgebrochen: {backup_job.get_backup_type_display()}",
+                details={"backup_job_id": str(backup_job.id)},
+            )
+        except Exception as audit_error:
+            # Don't fail the cancellation if logging fails
+            print(f"Warning: Could not log backup cancellation: {audit_error}")
         
         messages.success(request, f"Backup-Job wurde abgebrochen.")