Email delivery, hardened end to end

A big round of deliverability work landed:

• Double opt-in confirmation for new subscribers, with consent timestamp and IP recorded
• One-click unsubscribe plus a proper List-Unsubscribe header, backed by a real suppression list
• Broadcasts now send in throttled batches that respect your Amazon SES rate limit

Fewer bounces, cleaner lists, and broadcasts that no longer trip provider throttling.