Add the app billing features to the changelog

CHANGELOG.md

@@ -1,5 +1,23 @@
 # Changelog
 
+## Unreleased
+
+App billing features; they need an Anikeen ID release that supports them.
+
+- Free trials: `trial_days` / `trial_ends_at` (optional `trial_once`, `trial_payment_method` with
+  `TrialPaymentMethod::REQUIRED` / `OPTIONAL`) when creating subscriptions and checkout sessions;
+  `Subscription::isTrial()`.
+- Discounts per billed period: `discount` with `percent_off` or `amount_off`, `duration` and `label`.
+- Plan changes of running subscriptions: `AppSubscription::change()` (`apply: now` or
+  `period_end`) and `AppSubscription::cancelScheduledChange()`.
+- App orders: order lines with subscription, period, net amount and discount;
+  `AppOrders::invoicePdf()`. Confirmable orders of trusted apps (`confirm: true`).
+- Billing portal links that work without a login: `createBillingPortalLinkForUser()` and
+  `appBillingPortalUrl()`.
+- Webhook events `subscription.trial_will_end`, `subscription.discount_ended`,
+  `subscription.updated` and `subscription.renewed` (`EventType::SUBSCRIPTION_*`);
+  `data.reason` with `CancellationReason::TRIAL_ENDED_WITHOUT_PAYMENT_METHOD`.
+
 ## 3.1.0 (2026-11-05)
 
 - Add the scope `products:read` (`Scope::PRODUCTS_READ`) for read-only access to the product APIs