🔧 Configure Resend API key for production only

Ensure the Resend API key is set only in the production environment
to prevent accidental email delivery during development.

✨ Introduce deletion queued notification feature

Add functionality to notify users when their events are queued for deletion.
This includes new email templates, job scheduling, and updates to the event model
to track deletion status. Enhance tests to cover new scenarios.

✨ Add uploader count sorting and email preview

Implement uploader count sorting in events and add letter opener for email previews in development.