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* feat: Add foreign key to field `user_fk` of `EnterpriseCustomerUser` model. Part 5 of adding auth_user as a foreign key.
* Important: If you want to avoid downtime for a large EnterpriseCustomerUser table, you must ensure that no entries in that table are NULL (but the field remains nullable). To do this, deploy https://github.com/openedx/edx-enterprise/pull/2341 first and run the management command as defined there.