test: Enforce application layering using import linter (#36581)

* test: add import linters to promote clean dependencies

* docs: update a few comments / todos

openedx/core/djangoapps/content_staging/models.py

@@ -131,7 +131,6 @@ class UserClipboard(models.Model):
 
     def clean(self):
         """ Check that this model is being used correctly. """
-        # These could probably be replaced with constraints in Django 4.1+
         if self.user.id != self.content.user.id:
             raise ValidationError("User ID mismatch.")
         if self.content.purpose != CLIPBOARD_PURPOSE:

openedx/core/djangoapps/content_staging/views.py

@@ -81,6 +81,12 @@ class ClipboardEndpoint(APIView):
     def post(self, request):
         """
         Put some piece of content into the user's clipboard.
+
+        FIXME: This API needs to be deprecated and replaced by dedicated APIs
+        within each learning context (POST /course/foo/bar/copy, POST
+        /library/foo/bar/copy, etc.) We don't want to encode course- and
+        library-specific logic in content staging, and it shouldn't import
+        course or library modules.
         """
         # Check if the content exists and the user has permission to read it.
         # Parse the usage key: