alice starts a session on node-1
  session_start returned HTTP 200, session_id=9c28f913-3dd9-42a2-bd55-acbc7adf9447
alice writes 2 memories tagged with session_id
settle 6s for session-tagged fanout
bob lists on node-2 filtered by session_id=9c28f913-3dd9-42a2-bd55-acbc7adf9447
  bob sees 2 rows tagged session_id=9c28f913-3dd9-42a2-bd55-acbc7adf9447 (expected 2)