Add detailed debugging to backup_cancel function

🐛 Debug Enhancement:
- Added extensive print statements to track execution flow
- Identify exact location where 'User matching query does not exist' occurs
- Added traceback logging for better error diagnosis
- Isolated potential problem areas: user access, model saves, audit logging

This will help pinpoint where the user query error happens during backup cancellation.

app/stiftung/views.py

@@ -6013,9 +6013,13 @@ def backup_restore(request):
 def backup_cancel(request, backup_id):
     """Cancel a running backup job"""
     from stiftung.models import BackupJob
+    import traceback
     
     try:
+        print(f"DEBUG: Attempting to cancel backup job {backup_id}")
         backup_job = BackupJob.objects.get(id=backup_id)
+        print(f"DEBUG: Found backup job - ID: {backup_job.id}, Status: {backup_job.status}")
+        print(f"DEBUG: Created by: {backup_job.created_by}, Current user: {request.user}")
         
         # Only allow cancelling running or pending jobs
         if backup_job.status not in ['running', 'pending']:
@@ -6024,35 +6028,53 @@ def backup_cancel(request, backup_id):
         
         # Check if user has permission to cancel (either own job or admin)
         # Handle case where created_by might be None (user deleted)
+        print(f"DEBUG: Checking permissions - created_by: {backup_job.created_by}, is_staff: {request.user.is_staff}")
         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")
         
         # Mark as cancelled
+        print("DEBUG: About to mark job as cancelled")
         from django.utils import timezone
         backup_job.status = "cancelled"
         backup_job.completed_at = timezone.now()
+        
+        print(f"DEBUG: About to set error message with username: {request.user.username}")
         backup_job.error_message = f"Abgebrochen von {request.user.username}"
+        
+        print("DEBUG: About to save backup job")
         backup_job.save()
+        print("DEBUG: Backup job saved successfully")
         
         # Log the cancellation (with error handling)
         try:
+            print("DEBUG: About to log system action")
             from stiftung.audit import log_system_action
+            
+            print(f"DEBUG: About to call get_backup_type_display")
+            backup_type_display = backup_job.get_backup_type_display()
+            print(f"DEBUG: Backup type display: {backup_type_display}")
+            
             log_system_action(
                 request=request,
                 action="backup_cancel",
-                description=f"Backup-Job abgebrochen: {backup_job.get_backup_type_display()}",
+                description=f"Backup-Job abgebrochen: {backup_type_display}",
                 details={"backup_job_id": str(backup_job.id)},
             )
+            print("DEBUG: System action logged successfully")
         except Exception as audit_error:
+            print(f"ERROR in audit logging: {audit_error}")
+            print(f"ERROR traceback: {traceback.format_exc()}")
             # 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.")
         
     except BackupJob.DoesNotExist:
+        print(f"ERROR: Backup job {backup_id} not found")
         messages.error(request, "Backup-Job nicht gefunden.")
     except Exception as e:
+        print(f"ERROR: Unexpected error in backup_cancel: {e}")
+        print(f"ERROR traceback: {traceback.format_exc()}")
         messages.error(request, f"Fehler beim Abbrechen des Backup-Jobs: {e}")
     
     return redirect("stiftung:backup_management")