Error response format

All errors return a JSON object with three fields:

{
  "error": "error_type",
  "code": "error_code",
  "message": "Human-readable description"
}

Error codes

HTTP Status	Code	Description
`400`	`bad_request`	Missing or invalid query parameters
`401`	`unauthorized`	Missing or invalid API key
`402`	`payment_required`	Insufficient credits for this request
`404`	`not_found`	Endpoint does not exist
`429`	`rate_limited`	Rate limit exceeded (60 req/min)
`500`	`internal_error`	Server error (retry later)

A domain with no data returns 200 with data: [], not 404
The page-meta endpoint returns 200 with data: null when a URL is not found
Stub endpoints (link-velocity, new-links, lost-links) return 200 with status: "not_yet_available"
Failed requests do not deduct credits