Realtime SSE updates for mailbox views

Features

• Realtime SSE updates for mailbox views
• Enhanced suppression system with domain blocks and engagement tracking
• Onboarding page for new signups with API key, mailbox, and billing steps
• Demo video added to the landing page

Fixes

• Fixed DomainConnect signature encoding for Cloudflare compatibility
• Fixed classify job not triggering for inbound thread linking
• Fixed cooldown bypass validation for thread replies