feat(backend): bootstrap express app with error handling

packages/backend/src/app.ts

@@ -0,0 +1,33 @@
+import express, { type Express, type NextFunction, type Request, type Response } from 'express';
+import { ZodError } from 'zod';
+import { ApiError } from './lib/errors.js';
+
+export function createApp(): Express {
+  const app = express();
+  app.use(express.json({ limit: '5mb' }));
+
+  app.get('/api/health', (_req, res) => {
+    res.json({ ok: true });
+  });
+
+  // Routes mounted in later tasks.
+
+  app.use((err: unknown, _req: Request, res: Response, _next: NextFunction) => {
+    if (err instanceof ZodError) {
+      res.status(400).json({
+        error: { code: 'VALIDATION_ERROR', message: 'Invalid input', details: err.flatten() },
+      });
+      return;
+    }
+    if (err instanceof ApiError) {
+      res.status(err.status).json({
+        error: { code: err.code, message: err.message, details: err.details },
+      });
+      return;
+    }
+    console.error(err);
+    res.status(500).json({ error: { code: 'INTERNAL', message: 'Internal server error' } });
+  });
+
+  return app;
+}

packages/backend/src/index.ts

@@ -0,0 +1,7 @@
+import { createApp } from './app.js';
+
+const PORT = Number(process.env.PORT ?? 3000);
+const app = createApp();
+app.listen(PORT, () => {
+  console.log(`Backend listening on http://localhost:${PORT}`);
+});

packages/backend/src/lib/errors.ts

@@ -0,0 +1,20 @@
+export class ApiError extends Error {
+  constructor(
+    public status: number,
+    public code: string,
+    message: string,
+    public details?: unknown
+  ) {
+    super(message);
+  }
+
+  static notFound(what = 'Resource') {
+    return new ApiError(404, 'NOT_FOUND', `${what} not found`);
+  }
+  static validation(message: string, details?: unknown) {
+    return new ApiError(400, 'VALIDATION_ERROR', message, details);
+  }
+  static conflict(message: string) {
+    return new ApiError(409, 'CONFLICT', message);
+  }
+}