feat: add simple Express app with /hello endpoint and tests

tests/integration/app.test.js

@@ -0,0 +1,10 @@
+const request = require('supertest');
+const app = require('../../src/server');
+
+describe('GET /hello', () => {
+  it('should return Hello world', async () => {
+    const res = await request(app).get('/hello');
+    expect(res.statusCode).toBe(200);
+    expect(res.text).toBe('Hello world');
+  });
+});