Public API error codes

Public responses return compact JSON errors with one HTTP status code per failure.

Code	Meaning	Description
400	Bad request	The request parameters are invalid or incomplete.
401	Unauthorized	The X-API-Key header is missing or invalid.
404	Not found	The requested card, slug, set, or route target does not exist.
429	Rate limit exceeded	The current key exceeded its plan budget or request-per-second gate.
500	Internal server error	The API could not complete the request due to an internal failure.
503	Service unavailable	The backing database or service dependency is temporarily unavailable.