Campaign type.
When type is booster, there are two supported flows:
Flow 1 — draft parent + draft booster:
Create a draft parent campaign, then create a draft booster for it
(set boosterSettings.delay to control when it fires). Send the parent;
the booster is scheduled automatically based on the configured delay.
Flow 2 — already-sent parent:
Create and send a parent campaign first. Once it reaches sent status,
create the booster campaign (omit boosterSettings.delay) and use
sendingSettings.scheduledAt to set the exact send time. Then call
the send endpoint on the booster.
