List the link infrastructure keys in use

Operator-only: every dedicated link infrastructure key currently assigned to a company (Company.link_infrastructure_key), plus the shared cluster's key (mabl), each with its derived carrier FQDNs and holder company ids. Fleet tooling (mops) fans cluster-scoped operations out over this list, and the decommission runbook verifies a key has zero holders before removing its terraform clusters entry. See dedicated-link-infra.md in link/mablnet/docs.