Update subscription price

Set the authenticated creator's monthly subscription price, in USD cents (minor units).

⚠️ Side effect when increasing the price: all existing active subscribers are moved to a pending opt-in state for the new price. With forceOptIn: false (default) they keep auto-renewing at their current price until they accept the new one; with forceOptIn: true they are forced to opt in at the new price and their current auto-renewal is disabled. The response's subscribersMovedToReOptIn reports whether this happened. Decreasing the price has no effect on existing subscribers.

The new price must stay compatible with the creator's promotions: if any promotion's discount would take it below the platform minimum, the request is rejected until that promotion is deleted.