alice writes M1 + M2 + links M1→M2
  M1=5bb18963-fc36-429f-b09a-9800033874f2 M2=d2629102-9ac1-42cb-9da0-1d7413e9dcbf
settle 6s for link fanout
charlie queries /api/v1/links/M1 (forward)
charlie queries /api/v1/links/M2 (reverse)