Scale, reliability and edge cases: what breaks

Day 4 breaks the platform on purpose: a 10 million campaign burst tamed by batched, throttled, isolated ingestion; celebrity fan-out done asynchronously and keyed by recipient; provider rate limits enforced by an atomic Redis limiter; circuit breakers and provider failover; the edge case encyclopedia (tokens, bounces, DST, races); failure modes and graceful degradation with hysteresis; and cost-aware channel selection.