alice starts a session on node-1
  session_start returned HTTP 200, session_id=30321d29-a55d-4f88-a3e4-bb6167dda0f3
alice writes 2 memories tagged with session_id
settle 6s for session-tagged fanout
bob lists on node-2 filtered by session_id=30321d29-a55d-4f88-a3e4-bb6167dda0f3
  bob sees 2 rows tagged session_id=30321d29-a55d-4f88-a3e4-bb6167dda0f3 (expected 2)