Audit log system: every action, never lost, provably untouched

A complete audit log for a financial company at 50,000 events a second with 7-year retention: outbox capture, Kafka transport, a consumer with backpressure and a DLQ, tiered storage, hash chaining for tamper evidence, the edge cases (outbox growth, lag, broken chains, clock skew), real-world variants, and a banking mock interview. Every Week 3 pattern in one system.