alice starts a session on node-1
  session_start returned HTTP 200, session_id=2729f275-6450-4ef2-a43f-dfb2e549aad6
alice writes 2 memories tagged with session_id
settle 6s for session-tagged fanout
bob lists on node-2 filtered by session_id=2729f275-6450-4ef2-a43f-dfb2e549aad6
  bob sees 2 rows tagged session_id=2729f275-6450-4ef2-a43f-dfb2e549aad6 (expected 2)