Email delivery status now syncs correctly

Resend webhook events are now received and processed, enabling delivery and bounce status updates for emailed estimates.

Bug Fixes

Resend webhook events were being rejected with HTTP 500 errors because the webhook secret wasn't being synced from GitHub to Vercel during deploys. This prevented delivery and bounce status from updating on sent estimates. The secret is now included in the deployment sync, allowing webhooks to verify signatures and process events correctly.