Index Exchange: automate token refresh for INDEX_EXCHANGE_API_KEY #119

INDEX_EXCHANGE_API_KEY is a JWT bearer token that expires, and refreshing it is currently a documented manual step (docs/guides/troubleshooting.md, docs/guides/configuration.md). Once the token expires, every Index Exchange call fails with an auth error until an operator replaces the env var and restarts. Proposal: have IndexExchangeSSPClient obtain and refresh the token itself. Follow-up to #111.