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/routes/events.js

@@ -1,6 +1,7 @@
 const express = require('express');
 const { prisma } = require('../utils/db');
 const { authenticate } = require('../middleware/auth');
+const { getIO } = require('../socket');
 
 const router = express.Router();
 
@@ -566,6 +567,28 @@ router.post('/:slug/heats', authenticate, async (req, res, next) => {
       return userHeats;
     });
 
+    // Broadcast heats update to all users in event room
+    try {
+      const io = getIO();
+      const roomName = `event_${event.id}`;
+      io.to(roomName).emit('heats_updated', {
+        userId: req.user.id,
+        username: req.user.username,
+        heats: result.map(heat => ({
+          id: heat.id,
+          divisionId: heat.divisionId,
+          division: heat.division,
+          competitionTypeId: heat.competitionTypeId,
+          competitionType: heat.competitionType,
+          heatNumber: heat.heatNumber,
+          role: heat.role,
+        })),
+      });
+    } catch (socketError) {
+      // Don't fail the request if socket broadcast fails
+      console.error('Failed to broadcast heats update:', socketError);
+    }
+
     res.json({
       success: true,
       count: result.length,