alice writes M1 on node-1
bob writes M2 on node-2
  M1=0aa98b5f-0c55-495e-b5b7-1cc05103542b M2=173dda7a-e01c-449c-b3c8-e56f5bd5e64c
settle 5s for pre-link replication
alice links M1 -> M2 with relation=related_to
  link POST returned HTTP 201
settle 8s for link fanout
charlie queries links of M1 on node-3
  charlie sees M1->M2 link: 1 (expected >=1)