Put a cancelled day back

Clear a day's cancellation (cancelledAt); its slots, items and registrations are simply live again. Takes no body. Notifies nobody. A day that is not cancelled comes back unchanged, on any signup. On a day with Shifts of a repeating schedule, the Shifts the same cancel took down are reconnected too: restored counts them, and notRestored names any that could not come back (their Shift left the schedule, or their place is taken) with the reason; those stay cancelled and the day is restored regardless. A Shift cancelled earlier on its own stays cancelled and is not listed. Both are 0 / empty on an ordinary day. A day whose repeating-schedule Shifts were all cancelled one by one, with no day cancellation (cancelledAt null), has nothing to undo, so it is 409 conflict: put each Shift back with POST /signups/{id}/slots/{slotId}/reconnect. A day of another signup is 404.