Webhook ingestion for real-time receive + delivery tracking

[longevityboris]

Problem

Receiving is poll-only (sync must be called manually). Delivery lifecycle (delivered, bounced, opened, complained) is not tracked — last_event only stores the initial status.

Impact

• Agents miss emails until someone runs sync
• No way to know if a sent email actually reached the recipient
• Bounces and complaints are invisible — Resend auto-suppresses but we don't know
• Resend counts inbound quota (100/day free) — wasted without awareness

Proposal

1. Add webhook listen --port <port> command that starts a local HTTP server
2. Accept Resend webhook events: email.received, email.delivered, email.bounced, email.complained, email.opened, email.clicked
3. Verify webhook signatures (Resend uses Svix)
4. Store events in a new events table
5. Update last_event on messages when delivery events arrive
6. Add events list --message <id> to query delivery history

Priority: HIGH — identified by GPT-5.4 as #1 production gap