Add Express hello world example

lecture_3/hello-world-express/.prettierrc.json

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+{
+	"endOfLine": "lf",
+	"useTabs": true,
+	"printWidth": 120,
+	"tabWidth": 4
+}

lecture_3/hello-world-express/eslint.config.js

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+import js from "@eslint/js";
+import globals from "globals";
+import json from "@eslint/json";
+import { defineConfig } from "eslint/config";
+
+export default defineConfig([
+	{
+		ignores: ["node_modules/**", "package-lock.json"],
+	},
+	{
+		files: ["**/*.{js,mjs,cjs}"],
+		plugins: { js },
+		extends: ["js/recommended"],
+		languageOptions: { globals: globals.node, ecmaVersion: 2025 },
+		rules: {
+			indent: ["error", "tab"],
+			"linebreak-style": ["error", "unix"],
+			"no-unused-vars": ["error", { argsIgnorePattern: "^_" }],
+			quotes: ["error", "double"],
+			semi: ["error", "always"],
+		},
+	},
+	{ files: ["**/*.json"], plugins: { json }, language: "json/json", extends: ["json/recommended"] },
+]);

lecture_3/hello-world-express/hello-world.http

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+### Plain-text response
+GET http://localhost:3000/hello-world
+
+### JSON response
+GET http://localhost:3000/hello-world-json

lecture_3/hello-world-express/index.js

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+import express from "express";
+
+const app = express();
+const port = Number(process.env.PORT ?? 3000);
+
+// Return a plain-text response for the simplest HTTP endpoint example.
+app.get("/hello-world", (_request, response) => {
+	response.send("Hello world!");
+});
+
+// Return structured data and a value generated for each request.
+app.get("/hello-world-json", (_request, response) => {
+	response.json({
+		message: "Hello world!",
+		serverTime: new Date().toISOString(),
+	});
+});
+
+app.listen(port, () => {
+	console.log(`Server is running at http://localhost:${port}`);
+});

lecture_3/hello-world-express/package.json

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+{
+	"name": "hello-world-express",
+	"version": "1.0.0",
+	"type": "module",
+	"scripts": {
+		"start": "node index.js",
+		"lint": "eslint .",
+		"format": "prettier --write .",
+		"check": "npm run lint && prettier --check ."
+	},
+	"dependencies": {
+		"express": "^5.1.0"
+	},
+	"devDependencies": {
+		"@eslint/js": "^10.0.1",
+		"@eslint/json": "^1.2.0",
+		"eslint": "^10.0.0",
+		"globals": "^17.4.0",
+		"prettier": "^3.0.0"
+	}
+}