List available platform domains

Returns the **platform-managed** domains available for inbox creation, filtered by your plan: - **Free / Developer / Startup** — free-tier domains only. - **Growth / Enterprise** — free-tier + pro-tier domains. Results are cached for up to 5 minutes. New domains appear automatically without requiring a code change on your side. Tags on each domain: - `new` — recently added domain, shown for ~30 days. - `popular` — high-traffic domain. Domains expiring within 30 days include `expires_at` and `expires_in_days` as a heads-up to migrate inboxes. Healthy domains omit these fields entirely to keep the payload lean. > **Custom domains** (domains you own and verified) are managed via > `GET /v1/custom-domains` — this endpoint only returns platform domains.