Read the login bindings of an integration

Returns every login this integration has seen, unbound ones first — those are the only rows waiting on a decision.

The map fills itself: a login is recorded the first time an event names it, so the list is empty until the first delivery arrives and grows on its own afterwards. Nothing has to be declared in advance, and a login you have never received cannot be read here — but it can be written, see the PUT.

Only sources that name people have a login map at all; every other type answers 404. Today that is gitlab, whose merge request events name reviewers.