Document the items (subscription, period, net amount, discount), invoice
and metadata of orders read with the app's token, add
AppOrders::invoicePdf() for the invoice PDF and the opt-in
subscription.renewed event type with its period data.
Webhook::constructEvent() checks the Anikeen-Signature header (HMAC-SHA256
of "t.body" with the endpoint's secret, 5-minute tolerance) and returns
the event, like Stripe's helper.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>