Fix ES6 import/export mismatch in collaborate.js routes

- Import db module at top of file using ES6 import
- Remove dynamic require() call that was causing ReferenceError
- Database sessions now load correctly through GET /api/collaborate endpoint

backend/src/routes/collaborate.js

@@ -1,5 +1,6 @@
 import express from 'express'
 import collaborativeOrchestrator from '../services/collaborativeOrchestrator.js'
+import db from '../db/schema.js'
 
 const router = express.Router()
 
@@ -51,7 +52,6 @@ router.post('/', async (req, res) => {
  */
 router.get('/', (req, res) => {
   try {
-    const db = require('../db/schema.js').default
     const stmt = db.prepare(`
       SELECT
         id,