alice starts a session on node-1
  session_start returned HTTP 200, session_id=69f9e136-21f3-410e-80d3-1b12a5ac478c
alice writes 2 memories tagged with session_id
settle 6s for session-tagged fanout
bob lists on node-2 filtered by session_id=69f9e136-21f3-410e-80d3-1b12a5ac478c
  bob sees 2 rows tagged session_id=69f9e136-21f3-410e-80d3-1b12a5ac478c (expected 2)