alice starts a session on node-1
  session_start returned HTTP 200, session_id=f78a549e-db9a-4fdf-9a91-f59fdffc92c2
alice writes 2 memories tagged with session_id
settle 6s for session-tagged fanout
bob lists on node-2 filtered by session_id=f78a549e-db9a-4fdf-9a91-f59fdffc92c2
  bob sees 2 rows tagged session_id=f78a549e-db9a-4fdf-9a91-f59fdffc92c2 (expected 2)