Abstract the logic within send_digest_emails

[kevinrenskers]

To quote from the readme:

Create a cron job to send daily digests:

# Send daily digests at 9 AM
0 9 * * * cd /path/to/project && uv run ./manage.py send_digest_emails --frequency daily

But what if you don't want to create a cronjob? What if you use Celery or APScheduler or whatever else? While you can import and execute the Command, it would be better if the command would execute a method defined somewhere else, and this method could be called by Celery or APScheduler or whatever.

So, TL;DR: move the logic within send_digest_emails.py to a new file, so it can be easily executed by other task runners.