Webhooks for user management

Add user.created and user.updated webhook events to the existing webhook system.

Customers with custom integrations (e.g. chatbots, CRMs) need to react to user lifecycle events in real time. Without these, they can't keep external systems in sync with Featurebase user data.

We use SSO with multi-tenant companies, meaning a user can migrate between tenants and have their company linkage updated. So we need user.created to trigger downstream workflows (e.g., pulling enriched data from HubSpot) and user.updated to catch tenant migrations. Without these webhooks, our automation pipeline breaks.