The verbs an assistant can ask for go on the engines, not on a handler that
orchestrates repositories behind their backs. `dismissUpcoming` and
`snooze(id, minutesOverride)` move the ring slot and the backstop;
`dismissExpired` and `dismissAllExpired` move the timer's expiry registration.
Each of those is why they belong under the engine's mutex.
A snooze duration a caller sends is clamped to one hour, because that one is
the user's own request happening now. An alarm or timer marked to delete
itself is deleted as its cycle closes rather than left disabled.
makiolaj
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