docs(api-server): note that broadcast_status has no production caller (#1163)

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Summary

AppState::broadcast_status (api-server/src/state.rs) is the only producer for the TransactionStatusEvent channel that websocket::handle_socket reads from. A repo-wide grep shows it is invoked only from api-server/src/tests.rs — none of handlers.rs, websocket.rs, rpc.rs, or main.rs ever constructs or sends a TransactionStatusEvent.

The result: a real client connecting to /ws and subscribing to a tx_id will never receive a status_update message in production. The WebSocket subscribe / unsubscribe machinery (/ws, SubscribeMessage, MAX_SUBSCRIPTIONS_PER_CONNECTION, tx_subscribers, tx_status_tx) is fully wired up and well tested, but the production pipeline has no producer.

Wiring up a real producer (e.g. have handlers::simulate or a future submit / execute endpoint emit a TransactionStatusEvent once a transaction outcome is known) is a larger architectural change that is out of scope for a small fix. This change at least makes the stub state explicit so a future reader of websocket.rs in isolation is not misled into thinking the broadcast pipeline is a working feature.

Change

An extensive doc comment on broadcast_status capturing the stub status, the missing caller, the visible-but-silent production symptom, and a pointer to issue #1163 for follow-up. No behavior change, no tests, no API surface change — purely a maintainability comment so the next person who touches websocket.rs does not waste time debugging a pipeline that looks like it should work.

Closes

#1163