2.7.0

Added: a date can say when its tickets go on sale

presale_starts_at, nullable and UTC on disk like starts_at. A sold date has two instants that
matter — when the room opens and when the box office does — and the second is what a presale mailing
is timed against; "Der Vorverkauf hat gestartet" is only true from this moment on. Nullable because
a date without a presale date is the normal case, not an error: a placeholder like now() would
fire a mailing on a day nobody chose.

In the Control Panel the field sits in the Tickets section of the date form, shown in the event's
timezone like the two dates above. A presale after the date itself is refused, as a field error on
the form and as a model invariant — the controller check is the friendly path, not the authority.

Setting, moving or clearing the date fires OccurrencePresaleChanged, which carries
previousPresaleStartsAt because "the presale begins later now" is only worth sending with the old
day attached. Creation fires nothing new: a new date announces itself through OccurrenceScheduled,
presale date included, and a second event for the same row would let a listener fire twice on one
write. statamic-marketing can listen from here.