Persists feedback that flows over the event bus and serves the read/command side for a session. It is BOTH an event consumer (the write side — it persists TurnCompleted under each session's folder) AND the provider of the read/command FeedbackService for a session (the query side). The folder is derived from the event's session key by `rootFor`; the backing FeedbackService is the store-level swap point (File today, Database later), so changing where data lands needs no producer change.