Removes a shard once the server has acknowledged it.
Removes every shard of userID (logout).
Purges entries past retention (every user's), then returns up to limit of userID's shards, oldest first.
Persists a shard (idempotent per session+index).
Durable shard queue. No method throws: a storage failure is logged and the call resolves (empty for IRecordingShardStore.ListOldest).