Get Domain Availability

T-710: the single "can I create a project on this domain?" flag for the onboarding.

available is False when the domain is blacklisted (example / freemail / public company) OR already taken by 2+ other orgs — the same rule create_project enforces (409 domain_unavailable). The onboarding uses this read-only twin to render the "can't be used → contact support" screen up front instead of running the whole pipeline first.

T-973: an org that already holds an ACTIVE project on the domain always passes — create_project checks its own-project branch BEFORE the gate (project_exists 409 / enrich the stale-empty shell in place), so the probe must too, or the two disagree and the project's owner lands on the "contact support" dead-end (the onboarding can't shield this case itself: /user hides segmentless shells, so its existingProject guard misses exactly the stale-empty project).