Set a price (creator)
The collection must be public, then setprice_coins and a billing_period:
price_coins: 0 makes it free again (a public collection behaves normally, gated
only by the expose_* flags from Collections).
Subscribe (fan)
A fan subscribes — this charges the price immediately and grants access:See your subscriptions
once subscription expires_at is null (lifetime); for monthly it’s the
renewal date. Access to gated content is: owner, or free collection, or an
active subscription.