✨(backend) add creator field on document and allow filtering on it

We want to be able to limit the documents displayed on a logged-in user's
list view by the documents they created or by the documents that other
users created.

This is different from having the "owner" role on a document because this
can be acquired and even lost. What we want here is to be able to
identify documents by the user who created them so we add a new field.

src/backend/core/factories.py

@@ -56,6 +56,7 @@ class DocumentFactory(factory.django.DjangoModelFactory):
 
     title = factory.Sequence(lambda n: f"document{n}")
     content = factory.Sequence(lambda n: f"content{n}")
+    creator = factory.SubFactory(UserFactory)
     link_reach = factory.fuzzy.FuzzyChoice(
         [a[0] for a in models.LinkReachChoices.choices]
     )