alice starts a session on node-1
  session_start returned HTTP 200, session_id=03817bfc-9c8f-4427-8417-1e5b5ad4356b
alice writes 2 memories tagged with session_id
settle 6s for session-tagged fanout
bob lists on node-2 filtered by session_id=03817bfc-9c8f-4427-8417-1e5b5ad4356b
  bob sees 2 rows tagged session_id=03817bfc-9c8f-4427-8417-1e5b5ad4356b (expected 2)