refactor(websocket): remove unused broadcastToSession export

Remove the dead-code broadcastToSession function from
server/plugins/websocket.js. It was exported but never imported or
called anywhere in the codebase. Its dependencies
(getSessionConnections, addSessionConnection, removeSessionConnection)
remain since they are actively used for per-session connection
tracking.

Closes #40

server/plugins/websocket.js

@@ -43,16 +43,6 @@ export function removeSessionConnection(sessionId, ws) {
   }
 }
 
-export function broadcastToSession(sessionId, message) {
-  const conns = getSessionConnections(sessionId)
-  const data = JSON.stringify(message)
-  for (const ws of conns) {
-    if (ws.readyState === 1) { // OPEN
-      ws.send(data)
-    }
-  }
-}
-
 export default defineNitroPlugin((nitroApp) => {
   nitroApp.hooks.hook('ready', async () => {
     const server = nitroApp.h3App.server || nitroApp.h3App.nodeServer