Add email notifications, health checks, and admin improvements

Summary:

• Add subscriber email confirmation and unsubscribe flows
• Add automated health check service with URL monitoring
• Add JWT token verification endpoint and login validation
• Add admin settings panel (navbar, footer configuration)
• Support multiple navbar links and configurable footer
• Add service URL and auto_status fields for health checks
• Pass SMTP and app environment variables to Docker containers

Reviewers: ieli

Reviewed By: ieli

Differential Revision: https://devcentral.nasqueron.org/D3967