The JWKS document — GET /oauth/jwks. RS256 mode only.
In HS256 mode there is no public key: publishing an empty keys array would
invite a client to try verifying tokens it cannot verify, so this answers
404 and the metadata document omits jwks_uri entirely. Nothing about an
HMAC deployment needs this endpoint — the resource server holds the same
shared secret.
Public, cacheable, CORS-open. Only the public half of the key is ever rendered:
Tokens.jwks/1 runs JOSE.JWK.to_public/1, so a private key cannot leak here
even if one is misconfigured into :jwk.