Thanks @nobiot.
I consider line 589 here in org-roam-db-insert-link is a good place to start looking into this: org-roam/org-roam-db.el at main · org-roam/org-roam · GitHub
But then I realise that org-roam.db is only a cache layer over the data in the org files (right?). And could be rebuilt at any time, losing any externally added datetimes. So if I wished to store a created_at field for links then really I must also have it stored somewhere in the org files themselves. I think.
Which doesn’t seem particularly feasible. I would need to be able to annotate links - a bit like in Add Link-tags feature.