Billing Set Slots

Set the number of extra creator slots for ONE platform to exactly count (body: {"platform":"onlyfans"|"fansly","count":N}). Adjusts that platform's add-on item on the key's Stripe subscription. An increase is charged to the card on file right away, prorated for the rest of the billing period, and only applies if that payment succeeds (402 otherwise); the billing date never moves. A decrease is credited on the next invoice and is refused with 409 slots_in_use while active creators still occupy those slots. The account limit updates immediately. Any connected but inactive creator that now fits is activated at once (optionally name one as creator_id).