Live-comments during events

Design the live-comments / live-chat system for a streaming platform during a major event: a single stream can have 5M concurrent viewers, comment write bursts of 100k+ messages/sec, and viewers should see a lively but not overwhelming stream of comments within ~1–2s. Requirements: spam/abuse must be filterable in near-real-time, late joiners need recent context, and one mega-stream must not degrade other streams. Walk through ingestion, fan-out, and what you intentionally drop.