Cannot merge storage items

[mase76]

Hi!
When trying to merge storage items i get a server error and this traceback:

File "/home/partuniverse/env/lib/python3.9/site-packages/django/core/handlers/exception.py", line 34, in inner
    response = get_response(request)
  File "/home/partuniverse/env/lib/python3.9/site-packages/django/core/handlers/base.py", line 115, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "/home/partuniverse/env/lib/python3.9/site-packages/django/core/handlers/base.py", line 113, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/home/partuniverse/env/lib/python3.9/site-packages/django/contrib/auth/decorators.py", line 21, in _wrapped_view
    return view_func(request, *args, **kwargs)
  File "/home/partuniverse/env/lib/python3.9/site-packages/django/views/generic/base.py", line 71, in view
    return self.dispatch(request, *args, **kwargs)
  File "/home/partuniverse/env/lib/python3.9/site-packages/django/views/generic/base.py", line 97, in dispatch
    return handler(request, *args, **kwargs)
  File "/home/partuniverse/env/lib/python3.9/site-packages/django/views/generic/edit.py", line 142, in post
    return self.form_valid(form)
  File "/home/partuniverse/partuniverse/partuniverse/partsmanagement/views.py", line 581, in form_valid
    storageiitem.part.merge_storage_items(
  File "/home/partuniverse/partuniverse/partuniverse/partsmanagement/models.py", line 600, in merge_storage_items
    si2.delete()
  File "/home/partuniverse/env/lib/python3.9/site-packages/django/db/models/base.py", line 921, in delete
    collector.collect([self], keep_parents=keep_parents)
  File "/home/partuniverse/env/lib/python3.9/site-packages/django/db/models/deletion.py", line 224, in collect
    field.remote_field.on_delete(self, field, sub_objs, self.using)
  File "/home/partuniverse/env/lib/python3.9/site-packages/django/db/models/deletion.py", line 22, in PROTECT
    raise ProtectedError(
django.db.models.deletion.ProtectedError: ("Cannot delete some instances of model 'StorageItem' because they are referenced through a protected foreign key: 'VerifiedStock.storage_item'", <QuerySet [<VerifiedStock: ATTINY85-20SU; Werkstattmagazine->Werkstattmagazin 1 2022-08-11 21:43:46.630413+00:00>]>)

However the amount of one item is added to the other item, but the other one is not deleted.

[mase76]

#285 fixes this.