The `AlarmClock` contract says twice that an alarm or a timer created with
`SKIP_UI` should be removed once it has been dismissed. Without somewhere to
record that, a voice user's alarm list becomes a graveyard of one-shots nobody
asked to keep.
One column on each of `alarms` and `timers`, defaulting to 0, and a migration
that adds them. Existing rows keep the behaviour they have always had.
