Function createWebhookMiddleware

Create Express middleware for webhook verification

This middleware:

1. Extracts the signature from the request header
2. Verifies the signature using HMAC-SHA256
3. Parses the JSON payload
4. Attaches the parsed event to req.webhookEvent

The middleware expects the raw body to be available. Use with:

• express.raw({ type: 'application/json' }) for the webhook route
• Or ensure req.body is the raw string/Buffer before this middleware