Deliverability report, mailbox-provider breakdown and subscriber growth chart #39

Broadcast stats today are sent, open rate, click-through rate and recipient count (sequences/queries.ts). /overview has no rates, and the web app has no charts.

Proposal. Most of this follows https://github.com/marclou/mailcheap (CampaignReport.jsx, lib/overview-data.js, lib/providers.js).

• Per-broadcast and per-email report:
  • Delivered, opened, clicked and unsubscribed (divided by delivered).
  • Bounced and complained (divided by sent), colored amber at half the Auto-pause broadcasts and sequences on high bounce or complaint rates #25 pause threshold and red at the threshold.
• Breakdown by mailbox provider: Gmail, Outlook, Yahoo/AOL, iCloud, Proton, Other. Store the provider on each delivery at send time from the address domain, to keep aggregation cheap.
• Per-link clicks: link and index are already stored in email_events, but nothing adds them up yet.
• Overview: last 30 days against the previous 30 for sends, deliveries, opens, clicks, bounces and complaints. Show changes in % for counts and in percentage points for rates.
• Subscriber growth chart: daily joins and leaves in the viewer's time zone. Date a leave by when the unsubscribe or suppression happened, and fold contacts imported without a signup date into the starting count.
• Feedback-ID header (<sequenceId>:<emailId>:<teamId>:sendlit), so Gmail Postmaster Tools reports spam rates per campaign.
• Use shadcn charts (Recharts) in the web app.

Show human and machine numbers separately once bot filtering is in.