alice writes M1 on node-1
bob writes M2 on node-2
  M1=5bea8eb1-d3fc-4a1a-ad66-0bd56dcddd24 M2=25f148cc-9a08-4bf2-9f4e-c80bd6d5bbe2
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)