مخطط التسلسل وتصميم واجهة API

const express = require('express');const app = express();app.use(express.json()); const courses = [{ id: 1, title: 'تصميم الويب', capacity: 2 }];const enrollments = []; app.post('/api/enrollments', (req, res) => {  const { traineeId, courseId } = req.body;  if (!traineeId || !courseId)    return res.status(400).json({ error: 'traineeId و courseId مطلوبان' });  const course = courses.find(c => c.id === courseId);  if (!course) return res.status(404).json({ error: 'الدورة غير موجودة' });  const taken = enrollments.filter(e => e.courseId === courseId).length;  if (taken >= course.capacity)    return res.status(409).json({ error: 'الدورة مكتملة' });  const e = { id: enrollments.length + 1, traineeId, courseId, status: 'pending' };  enrollments.push(e);  res.status(201).json(e);}); app.listen(3000);