Add delivery status webhooks for campaign completion reporting

## Problem When a campaign is marked as "sent" in the UI, there's currently no real-time visibility into delivery failures that occur hours later. Issue #294 documents a scenario where campaigns show as sent while ~50% of emails fail asynchronously. This creates a data integrity gap: the campaign state doesn't reflect actual delivery outcomes, and users have no programmatic way to track which contacts received emails vs. which bounced/failed. ## Why This Matters 1. **User trust**: Users believe campaigns are delivered when marked "sent," but failures are silently occurring 2. **Missing data**: No audit trail of which sends actually succeeded, blocking troubleshooting and compliance 3. **Integration gaps**: External systems (CRMs, analytics platforms) can't react to delivery failures in real-time 4. **Operational blindness**: Support teams can't proactively alert users to partial campaign failures ## Current State Plunk tracks campaign delivery through AWS SES, but the webhook/event system doesn't expose: - Per-email delivery status updates (bounce, complaint, send confirmation) - Campaign-level aggregated delivery reports - A way for users to subscribe to delivery events via webhooks ## Proposed Solution Implement campaign delivery status webhooks that fire when: 1. An email in a campaign bounces or receives a complaint 2. A campaign reaches 100% delivery attempt completion 3. Delivery status changes in bulk (e.g., 10+ bounces from a single campaign) Webhooks should include: - Campaign ID and name - Contact ID and email address - Delivery status (sent, bounce, complaint, etc.) - AWS SES response metadata - Timestamp Users can subscribe to these webhooks via the UI or API to trigger notifications, update contact states, or log delivery metrics. ## Analogy SendGrid's [Event Webhook](https://docs.sendgrid.com/for-developers/tracking-events/event) and Resend's [webhook](https://resend.com/docs/api-reference/webhooks) both provide real-time delivery events, allowing users to react instantly to failures instead of discovering them in reports hours later. ## Acceptance Criteria - [ ] Webhook subscription mechanism exists (via UI and API) - [ ] Webhooks fire for bounce, complaint, and delivery confirmation events - [ ] Campaign shows aggregate delivery stats (sent, bounced, failed) that update in real-time - [ ] Documentation explains webhook payload and setup - [ ] E2E test verifies webhook delivery for a test campaign --- *Contributed by [Klement Gunndu](https://github.com/KlementMultiverse)*