WebSocket upgrade endpoint.

Connect with a WebSocket client and exchange JSON messages using the WsRequest/WsResponse schemas. The endpoint can also be explored through /test-websocket.html on a Blockbook instance.

During a backend reorg, new-block notifications can contain a block hash at the same or even smaller height.

Load estimate: Variable; idle connections are low, request methods cost roughly like their REST equivalents, and subscriptions grow with connection count, subscribed addresses, and event frequency.