Obtain a Twitch app access token using client credentials

Steps

1. Register your application in the Twitch Developer Console to obtain a client_id and client_secret.
2. Send a POST request to the Twitch OAuth token endpoint with client_id, client_secret, and grant_type=client_credentials as form-encoded body parameters.
3. Parse the access_token and expires_in values from the JSON response.
4. Store the token securely and include it as a Bearer token in the Authorization header of subsequent Helix API calls, alongside the Client-Id header.
5. Implement proactive token refresh: when the token is near expiry (or on receipt of a 401), request a new token rather than waiting for failures.