feat(api): schedule scavenge-unused management command daily

Running during daytime so that any potential errors have a higher
chance of being noticed quickly. Load is neglibile.

api/cronhook/crontab

@@ -1,2 +1,3 @@
 */5 * * * * /usr/local/bin/python3 -u /usr/src/app/manage.py chores >> /var/log/cron.log 2>&1
 */5 * * * * /usr/local/bin/python3 -u /usr/src/app/manage.py check-slaves >> /var/log/cron.log 2>&1
+7 11 * * * /usr/local/bin/python3 -u /usr/src/app/manage.py scavenge-unused >> /var/log/cron.log 2>&1