Imported layers are indirectly attached to projects via a Layer_Version record.

When a delete is performed on a project, Django will perform the deep delete on all related records, but it cannot find these related imported layer objects. This results in orphaned layers that can block re-adding that layer subsequently (because names must be unique).

The project delete method needs to capture those layers to delete before the project is deleted, and then explicitly delete then (as long as they are not referenced by other projects).