Celebrity profile cache stampede

Design the caching layer for a celebrity-account read path on a social network where a single profile/timeline key can be requested 500k times/second during a viral moment. Reads dominate writes 10000:1, but a profile edit must reflect within a few seconds. You have a Redis fleet in front of a sharded Postgres. Walk through how you keep a single hot key from melting one shard, and how you avoid a cache stampede when that key expires.