alice starts a session on node-1
  session_start returned HTTP 200, session_id=c2249f6d-4fd0-4d8d-a2ce-08452baac658
alice writes 2 memories tagged with session_id
settle 6s for session-tagged fanout
bob lists on node-2 filtered by session_id=c2249f6d-4fd0-4d8d-a2ce-08452baac658
  bob sees 2 rows tagged session_id=c2249f6d-4fd0-4d8d-a2ce-08452baac658 (expected 2)