alice starts a session on node-1
  session_start returned HTTP 200, session_id=4e9bfd87-f1c6-4a54-a43d-93715a94199c
alice writes 2 memories tagged with session_id
settle 6s for session-tagged fanout
bob lists on node-2 filtered by session_id=4e9bfd87-f1c6-4a54-a43d-93715a94199c
  bob sees 2 rows tagged session_id=4e9bfd87-f1c6-4a54-a43d-93715a94199c (expected 2)