alice starts a session on node-1
  session_start returned HTTP 200, session_id=5b624a5a-7985-4aa0-9f04-6eb613be010f
alice writes 2 memories tagged with session_id
settle 6s for session-tagged fanout
bob lists on node-2 filtered by session_id=5b624a5a-7985-4aa0-9f04-6eb613be010f
  bob sees 2 rows tagged session_id=5b624a5a-7985-4aa0-9f04-6eb613be010f (expected 2)