every six hours, between hourly and daily

Anchored to the clock — midnight, six, noon, six — rather than to whenever the
schedule was saved, which is the whole reason these are cron expressions.

Verified the catch-up check reads its period as six hours, since that is what
decides whether a run was missed while the machine was down.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

web/BackupsRow.tsx

@@ -41,6 +41,8 @@ type Settings = {
 /** Every schedule anybody has actually wanted, and their expressions. */
 const CHOICES = [
   { label: 'Hourly', cron: '0 * * * *' },
+  // Midnight, 6am, noon and 6pm — anchored to the clock rather than to whenever this was saved.
+  { label: 'Every 6 hours', cron: '0 */6 * * *' },
   { label: 'Daily, 3am', cron: '0 3 * * *' },
   { label: 'Weekly, Sun 3am', cron: '0 3 * * 0' },
   { label: 'Monthly, 1st 3am', cron: '0 3 1 * *' },