Activity Feed: Compaction and Rebuilds

Suppose the feed store is lost at 02:00 tonight, or a bug in the feed writers is found that has stored wrong entries for three days. From what would you rebuild the feeds of 3 million members, and how long would the rebuild take?

The feed store is a derived copy, so the answer must already exist in Kafka. A rebuild needs two inputs: the activities that belong in the feeds, and the current list of who follows whom. Each input is held by a different topic, and each topic keeps its data under a different cleanup policy.