subscribe to MailerLite webhook events for subscribers and campaigns

Steps

1. POST to https://connect.mailerlite.com/api/webhooks with name, events (array), url, and enabled
2. Set batchable: true if subscribing to campaign.open, campaign.click, or subscriber.deleted — these require batching
3. Verify each incoming request using the Signature header: HMAC-SHA256 of the JSON payload using the webhook's secret
4. Respond with a 2xx status within 3 seconds; offload heavy processing to a background job to avoid timeout-triggered retries
5. List/update/delete webhooks via GET/PUT/DELETE on /api/webhooks/{webhook_id}