Cancel one day of a signup

Call a day off, executed immediately: there is no dry run. Everyone holding anything on the day is emailed (and texted when they gave a phone number and SMS consent); message is added to that notification. Nothing is deleted — slots, items and registrations are kept. The write depends on the day. A day with Shifts of a repeating schedule has each of its live Shifts cancelled (the Shifts running on that day, wherever the schedule first put them); the response is kind: "series_date" with cancelled, affectedParticipants (registrations), dayCancelledAt and notified. The whole day is called off: it reads cancelledAt set (the same instant dayCancelledAt returns and the cancelled Shifts share), so a one-off slot on it is off too (nothing is deleted), and everyone holding anything on it is told. It is undone with POST /signups/{id}/days/{dayId}/uncancel, which reconnects exactly the Shifts this call cancelled; a Shift can also be put back alone with POST /signups/{id}/slots/{slotId}/reconnect. Any other day — on an ordinary signup, or one with no Shift of the repeating schedule — is itself cancelled; the response is kind: "day" with affectedSignups (people) and notified, and it is undone with POST /signups/{id}/days/{dayId}/uncancel. A repeat cancel tells nobody again. The account behind the credential must have a verified email, or the call is 403 forbidden. A day of another signup is 404.