write cron job

[emnurul]

• [x] have a cron job which looks for previews which have not been used in a long time, eg 3 months and delete them
• [x] have a cron job which looks for previews which are over a certain age (say 2 weeks) and have been previewed in that time and refresh them. We don't want to simply delete them because we'd prefer to show an old but fast preview over a slow but newer one.