Add a session cleanup task

**TL;DR**: Remove expired sessions from the database on a regular basis.

Note that Django [does not automatically clear expired sessions](https://docs.djangoproject.com/en/3.1/topics/http/sessions/#clearing-the-session-store) so they just accumulate in the database. We can add a Celery task ourselves or use [django-session-cleanup](https://pypi.org/project/django-session-cleanup/).

Read Django's session-clearing documentation and the django-session-cleanup package, then inspect how this repository handles Celery tasks and session storage. Done means expired sessions are removed from the database on a regular schedule, with coverage confirming the cleanup behavior.

Written by the indexing model from the issue text.