feat: add off-the-shelf middleware example

Author: enjoy15

middleware-off-the-shelf/README.md

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+# Off-the-shelf middleware example
+
+This Express app replaces the custom logger with the `morgan` middleware.
+The `requireApiKey` middleware is still custom.
+
+## Run locally
+
+```bash
+npm install
+npm start
+```
+
+Set a custom API key (optional):
+
+```bash
+API_KEY=my-secret npm start
+```
+
+## Try it
+
+```bash
+curl http://localhost:3001/health
+curl http://localhost:3001/messages
+curl -X POST http://localhost:3001/messages \
+  -H "Content-Type: application/json" \
+  -H "x-api-key: cyf" \
+  -d '{"user":"Azin","message":"Hello"}'
+```

middleware-off-the-shelf/middlewares/requireApiKey.js

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+module.exports = function requireApiKey(req, res, next) {
+  const expectedKey = process.env.API_KEY || "cyf";
+  const providedKey = req.header("x-api-key");
+
+  if (providedKey !== expectedKey) {
+    return res.status(401).json({ error: "Invalid API key" });
+  }
+
+  next();
+};