A lobby a client session was in.

Written once per (session, lobby) pair with on_conflict: :nothing, so repeated batches from the same run cost an ignored insert rather than a read-modify-write that two nodes could race.

Keyed by client_session_id (the client-generated string) rather than by the session row's id, so a batch can record its lobbies without first resolving the row. See the migration for why this is a table and not an array column.