alice starts a session on node-1
  session_start returned HTTP 200, session_id=70dc33e5-bd46-4d19-b7b6-f567da2d7807
alice writes 2 memories tagged with session_id
settle 6s for session-tagged fanout
bob lists on node-2 filtered by session_id=70dc33e5-bd46-4d19-b7b6-f567da2d7807
  bob sees 2 rows tagged session_id=70dc33e5-bd46-4d19-b7b6-f567da2d7807 (expected 2)