Add email bounce/retry handling

[aultra]

Description

Implement automatic bounce detection and retry logic for the email sending infrastructure.

Requirements

• Detect bounced emails (NDR/DSN handling from Gmail API)
• Categorize bounces (soft vs. hard bounce)
• Automatic retry for soft bounces (with backoff)
• Mark hard-bounced addresses to prevent future sends
• Surface bounce status in email logs UI (if applicable)

Infrastructure

Current email system logs sends to email_logs table with status (pending, sent, failed) and error messages, but has no automatic bounce detection or retry mechanism.

Files

• lib/services/email_service.dart — Dart email service
• supabase/functions/send-email/index.ts — Edge function
• supabase/functions/_shared/email-smtp.ts — Gmail API implementation