alice starts a session on node-1
  session_start returned HTTP 200, session_id=1bd2e96c-f629-4fa7-891a-23dda27a2a82
alice writes 2 memories tagged with session_id
settle 6s for session-tagged fanout
bob lists on node-2 filtered by session_id=1bd2e96c-f629-4fa7-891a-23dda27a2a82
  bob sees 2 rows tagged session_id=1bd2e96c-f629-4fa7-891a-23dda27a2a82 (expected 2)