Increase backup frequency (snippets, 29c6fcc)

Now that we are targetting a larger backup volume, switch to more frequent backups. This raises the count from 4 times per day (every 6 hours) to 12 times per day (every 2 hours).

This is the practical limit for backup frequency as each backup currently takes about 1 hour and 40 minutes.

Signed-off-by: Greg Hurrell <greg@hurrell.net>