alice starts a session on node-1
  session_start returned HTTP 200, session_id=df614fa6-b756-47f6-b331-32a0c3d69eb5
alice writes 2 memories tagged with session_id
settle 6s for session-tagged fanout
bob lists on node-2 filtered by session_id=df614fa6-b756-47f6-b331-32a0c3d69eb5
  bob sees 2 rows tagged session_id=df614fa6-b756-47f6-b331-32a0c3d69eb5 (expected 2)