US Treasury auction results with the tail and bidder split, plus economic
releases scored against forecast. Free tier needs no credential at all:
https://api.helious.io/v1/rates returns 200 with no key.
Auth: No (keyless tier; OAuth 2.1 and bearer keys are optional upgrades)
HTTPS: Yes
CORS: Yes (access-control-allow-origin: * verified on /v1)
Docs: https://helious.io/developers/
