How should a browser connect to the live transcription API, and what can I recover after a disconnect?

Keep the API key on your server. Have the server create the session and issue a short-lived, single-use ticket for the browser connection. A disconnect is not a normal finish. Check the original sessionId; get a new ticket if it can resume, or use finish on that session to end it. Then use the transcriptionId to read confirmed text. PARTIAL is a terminal partial result: it will not become COMPLETED by waiting and cannot use a download that requires COMPLETED. Reconnecting cannot recover audio that was never sent.