Add webhook signature verification and payload types

[brd6]

Notion webhooks sign each delivery with an X-Notion-Signature header (HMAC-SHA256 of the body with the verification token). The SDK offers nothing for this, so consumers verify signatures and parse event payloads by hand.

To add, following the JS SDK's webhooks.ts:

• a verifier that checks X-Notion-Signature against the raw request body (constant-time comparison)
• typed event payloads for the delivered events (page/data source/comment created, updated, deleted, etc.)

Docs: https://developers.notion.com/reference/webhooks