alice starts a session on node-1
  session_start returned HTTP 200, session_id=cc1d28ac-5584-4599-8de0-65f0bdb06f4e
alice writes 2 memories tagged with session_id
settle 6s for session-tagged fanout
bob lists on node-2 filtered by session_id=cc1d28ac-5584-4599-8de0-65f0bdb06f4e
  bob sees 2 rows tagged session_id=cc1d28ac-5584-4599-8de0-65f0bdb06f4e (expected 2)