feat: add Socket.IO heats_updated broadcast event

- Export getIO function from socket module
- Broadcast heats_updated event when user updates their heats
- Event includes userId, username, and updated heats array
- Non-blocking broadcast (won't fail request if socket fails)

backend/src/socket/index.js

@@ -5,8 +5,18 @@ const { prisma } = require('../utils/db');
 // Track active users in each event room
 const activeUsers = new Map(); // eventId -> Set of { socketId, userId, username, avatar }
 
+// Global Socket.IO instance
+let io;
+
+function getIO() {
+  if (!io) {
+    throw new Error('Socket.IO not initialized');
+  }
+  return io;
+}
+
 function initializeSocket(httpServer) {
-  const io = new Server(httpServer, {
+  io = new Server(httpServer, {
     cors: {
       origin: process.env.CORS_ORIGIN || 'http://localhost:8080',
       credentials: true,
@@ -348,4 +358,4 @@ function initializeSocket(httpServer) {
   return io;
 }
 
-module.exports = { initializeSocket };
+module.exports = { initializeSocket, getIO };