Public username check. Returns availability and up to 5 free suggestions. 30 requests per minute per IP. Responses include RateLimit-Limit, RateLimit-Remaining, and RateLimit-Reset (seconds until the window ends). HTTP 429 rate_limited includes Retry-After, retry_after_seconds, and resets_at, and RateLimit-Reset matches Retry-After. A lookup that cannot be completed returns HTTP 503 username_check_unavailable. If the limit cannot be checked, the response is HTTP 503 rate_limit_unavailable. Both 503 responses include Retry-After, retry_after_seconds, and resets_at.