fix(backend): correct relative imports in socket module

- socket.js lives in src/, so imports must be ./middleware/auth.js
  and ./config/db.js (was ../.., which broke server startup)

backend/src/socket.js

@@ -1,6 +1,6 @@
 import { Server } from 'socket.io';
-import { verifyToken } from '../middleware/auth.js';
-import { query } from '../config/db.js';
+import { verifyToken } from './middleware/auth.js';
+import { query } from './config/db.js';
 
 /**
  * Real-time collaboration protocol (see docs/API.md):