Server-Sent Events stream of scan payment status changes for a single PaymentTransaction.

Emits data: {json}\n\n frames carrying {type: "status" | "done" | "timeout", ...}.
Heartbeat comments (: ping\n\n) are emitted every 15 seconds.

The stream closes on the first terminal status (success / failed / cancelled /
abandoned / refunded) or after 10 minutes, whichever comes first.

Browsers cannot attach Bearer headers to native EventSource. Consumers should use
fetch + a streaming reader and parse the SSE chunks manually.