Auto-pause broadcasts and sequences on high bounce or complaint rates

SendLit already ingests bounce and complaint events from the supported ESPs, but nothing acts on the rates.

Add per-team thresholds, for example pause at 2% bounces or 0.08% complaints once at least 100 emails have been sent.
Pause the affected broadcast or sequence and notify the team.
Show the current rates in the UI and allow an explicit resume.

Builds on #6 (bounce/complaint handling). Inspired by https://github.com/marclou/mailcheap.

Extra rules worth adopting from https://github.com/marclou/mailcheap (lib/sending-rules.js):

Check the rates before each batch, not only when the send finishes, using the thresholds above (2% bounces / 0.08% complaints after at least 100 sent).
Also pause after 5 consecutive send failures, and on fatal ESP errors: account suspended, sending paused, MAIL FROM/domain not verified, limit exceeded. Throttling errors should retry rather than pause.
Count re-engagement ("last chance") emails from Dormant contacts segment and re-engagement sequence #29 toward the rates.
Alert with a cooldown (an atomic upsert on an alert key) so one bad campaign doesn't flood the team's inbox.
Showing the rates needs SES feedback (Amazon SES bounce and complaint ingestion via SNS #33) and the deliverability report (Deliverability report, mailbox-provider breakdown and subscriber growth chart #39).

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