user invites: backend: no collaborators found when "sync all repositories" chosen

Created by: slimsag

Investigating #31277 (closed) I found that there is a bug in the backend implementation: "sync all repositories" always finds zero collaborators right now.

This seems to be because we need to use the mirroring API in that case, since a full list is not composed on the backend but rather progressively found via the syncing backend.