# description
GET /api/domains/subdomain/check — whether a free subdomain label can be claimed. Read-only; safe to call on every keystroke as the user edits the label.

# properties.available.description
True when the label passes validation and nobody holds it yet.

# properties.reason.description
Why an unavailable label was rejected — show it verbatim. Either a format rule ('At least 3 characters', 'Use a single label (no dots)', 'Use letters, numbers and hyphens only', 'That subdomain is reserved') or 'Already taken'.

# properties.domain.description
The full hostname the label resolves to. Absent when the label is malformed enough that no hostname could be formed.