List databases available to a project (owned + shared + company-visible).

The third branch — a company-level dataset (project_id IS NULL) whose source_kind is in COMPANY_VISIBLE_SOURCE_KINDS, today the platform-maintained Users / User Groups directory datasets — is grant-gated, so this endpoint filters it per caller. Project VIEWER on the project is NOT enough to see one: the directory datasets are authorized by explicit per-user grant (domains/attribute_sync/CLAUDE.md), and this listing feeds the step editor's dataset picker, so an ungated row here would advertise the company's member emails and attribute values to every project member.