[QOTD-18] Implement global error-handling middleware

Add src/middleware/error.ts with a four-argument Express error handler
that logs errors server-side via console.error and responds with HTTP
500 and a generic { error: 'Internal server error' } body, never
leaking stack traces to the client. Register it last in a new
src/server.ts entry point.

src/middleware/error.ts

@@ -0,0 +1,11 @@
+import type { NextFunction, Request, Response } from 'express';
+
+export function errorHandler(
+  err: unknown,
+  _req: Request,
+  res: Response,
+  _next: NextFunction
+): void {
+  console.error(err);
+  res.status(500).json({ error: 'Internal server error' });
+}

src/server.ts

@@ -0,0 +1,10 @@
+import express from 'express';
+import { errorHandler } from './middleware/error';
+
+const app = express();
+
+app.use(express.json());
+
+app.use(errorHandler);
+
+export default app;