Errors, limits, and retries

Errors use an error object with code, message, and requestId. Responses also carry X-Request-Id.

Status	Meaning	Action
`400` / `422`	Invalid input	Fix the request; do not retry unchanged.
`401`	Missing or invalid key	Check `X-Api-Key` without logging it.
`402`	Account cannot fund this operation	Resolve the account state.
`404`	Request is absent or belongs to another account	Check the request ID and key.
`429`	Rate limit exceeded	Wait for `Retry-After`, then retry with jitter.
`502` / `503`	Temporary upstream or service failure	Retry with exponential backoff.

Reuse the same Idempotency-Key only for an identical write. Never rotate the key merely because a response timed out.