🐛 Fixing verifyWebhook to return the payloads PDFMonkey actually sends

[simonc]

Summary

verifyWebhook() rejected every real webhook. After the signature check passed, it required a { type, data, timestamp } envelope, but the backend sends the body as-is (DocumentGenerations::WebhookBroadcastingJob, SendQuotaNotificationsJob):

• documents.generation.success / documents.generation.failure: { "document": <document card JSON> }
• quota.warning: { period_start, period_end, available_documents, threshold }

The event names document.done / document.error never existed either.

Changes

• verifyWebhook() keeps the signature and timestamp checks, then returns the parsed body as WebhookPayload (DocumentWebhookPayload | QuotaWarningWebhookPayload). The only shape check left is that the body is a JSON object.
• DocumentWebhookPayload.document reuses DocumentCard. The event type isn't in the body, so callers check 'document' in payload and payload.document.status.
• Removed WebhookEvent, WebhookEventType, DocumentDoneEvent(Data), DocumentErrorEvent(Data) and UnknownWebhookEvent. This is a minor bump: the types change, but the old shape never worked against the live API.
• Updated the README example and recipes, and rewrote the test fixtures to the real shape.

Notes

• This touches CHANGELOG.md "Unreleased" and src/webhooks.ts, which ✨ Adding search and multi-status filters to documentCards.list #19 also edits. Whichever merges second needs a small rebase.

Testing

• pnpm test (212 passed), pnpm lint, tsc --noEmit and pnpm build pass.