alice starts a session on node-1
  session_start returned HTTP 200, session_id=5e75b402-5f40-4cd8-b36c-3c5f8a4d9195
alice writes 2 memories tagged with session_id
settle 6s for session-tagged fanout
bob lists on node-2 filtered by session_id=5e75b402-5f40-4cd8-b36c-3c5f8a4d9195
  bob sees 2 rows tagged session_id=5e75b402-5f40-4cd8-b36c-3c5f8a4d9195 (expected 2)