Subscribe to inbox notifications over SSE.

Opens a Server-Sent Events stream that emits one frame per batch of newly-available messages on the inbox. The connection stays open until the JWT expires, at which point the server closes it.

Two kinds of frames are emitted:

• Data frames (data: {...}\n\n) signal availability only, they do not include message content. Consumers should call the lease endpoint to retrieve messages. The JSON payload is described by the response schema below.
• Heartbeat frames (: keep-alive\n\n) are SSE comment lines sent when no notification has arrived within the heartbeat interval. They are ignored by EventSource clients and exist only to keep intermediaries from closing the connection as idle.