BearerChallengeUnauthorizedHandler
An UnauthorizedHandler that includes a WWW-Authenticate: Bearer challenge on 401 responses.
Enable by setting bearerChallenge = true in the jwt { } block (optional realm, default "API"). The response body is empty; only the challenge header is sent. When failure is an AuthenticationResult.Failure, the header includes error="invalid_token"; when the caller was simply unauthenticated, failure is null and no error attribute is included.