alice starts a session on node-1
  session_start returned HTTP 200, session_id=8bc8beb7-a6e2-4bf5-a688-4072ca31aca9
alice writes 2 memories tagged with session_id
settle 6s for session-tagged fanout
bob lists on node-2 filtered by session_id=8bc8beb7-a6e2-4bf5-a688-4072ca31aca9
  bob sees 2 rows tagged session_id=8bc8beb7-a6e2-4bf5-a688-4072ca31aca9 (expected 2)