alice writes M1 + M2 + links M1→M2
  M1=dcbe7e82-2a1d-466c-96f4-23f0d93ea38f M2=ece52486-547f-41f5-886e-94a5c81d7ebc
settle 6s for link fanout
charlie queries /api/v1/links/M1 (forward)
charlie queries /api/v1/links/M2 (reverse)