The API reference now matches what the API does

Several places where the API reference promised one thing and the API did another are now settled, each on the side that was correct.

• Listing pricing functions honours the built-in filter. Asking for only built-in functions, or only your own, previously returned the full list with no indication the filter had been dropped. An unrecognised value is now rejected instead of ignored.
• The default marketplace commission is 30%, which is what the reference, the stored default, and the payout maths already said. Listings created through the API were getting 20%. Setting the rate explicitly works as before.
• Listings created before this change keep the rate they were created with. A listing that was created without setting a rate took the old 20% default and still does, while the published default is now 30%; one where you set the rate yourself is unaffected and always was. Nothing was changed retroactively. If you have a listing on the older default and want it moved, contact support@last-price.ai.
• The pricing object returned with a variant carries whatever fields that variant's metadata defines, and the reference now says so and documents how to set it. Putting features in variant metadata is the supported way to return real feature copy. Metadata that reuses plan, price or features has to match the type the response declares, and an experiment whose metadata would break the response is now rejected when you create it rather than producing an invalid response later.
• The inbound mail webhook documents the signature headers it verifies, so generated clients and security reviews see it as authenticated.