Pusher change history: US3 cluster - major outage

Pusher reported a major US3 outage affecting Channels presence channels, Channels REST API and Channels WebSocket client API, with latency, 503 errors and full service downtime. It started at 2025-10-20 18:36 UTC and resolved at 2025-10-20 20:41 UTC, lasting 2 h 5 min. The postmortem identifies Redis cluster startup failures; Pusher reported implementing a fix.