feat(backend): bootstrap express app with error handling

packages/backend/package.json

@@ -0,0 +1,39 @@
+{
+  "name": "@flashcard/backend",
+  "version": "0.1.0",
+  "private": true,
+  "type": "module",
+  "main": "dist/index.js",
+  "scripts": {
+    "dev": "tsx watch src/index.ts",
+    "build": "tsc -p tsconfig.json",
+    "start": "node dist/index.js",
+    "typecheck": "tsc --noEmit",
+    "test": "vitest run",
+    "test:watch": "vitest",
+    "db:generate": "drizzle-kit generate",
+    "db:migrate": "tsx src/db/migrate.ts",
+    "db:seed": "tsx src/db/seed.ts"
+  },
+  "dependencies": {
+    "@flashcard/shared": "*",
+    "better-sqlite3": "^11.0.0",
+    "drizzle-orm": "^0.33.0",
+    "express": "^4.19.0",
+    "multer": "^1.4.5-lts.1",
+    "xlsx": "^0.18.5",
+    "zod": "^3.23.0"
+  },
+  "devDependencies": {
+    "@types/better-sqlite3": "^7.6.0",
+    "@types/express": "^4.17.0",
+    "@types/multer": "^1.4.0",
+    "@types/node": "^20.0.0",
+    "@types/supertest": "^6.0.0",
+    "drizzle-kit": "^0.24.0",
+    "supertest": "^7.0.0",
+    "tsx": "^4.16.0",
+    "typescript": "^5.5.0",
+    "vitest": "^2.0.0"
+  }
+}

packages/backend/src/app.ts

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+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

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+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

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+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);
+  }
+}

packages/backend/tsconfig.json

@@ -0,0 +1,10 @@
+{
+  "extends": "../../tsconfig.base.json",
+  "compilerOptions": {
+    "outDir": "dist",
+    "rootDir": "src",
+    "module": "NodeNext",
+    "moduleResolution": "NodeNext"
+  },
+  "include": ["src/**/*"]
+}

packages/backend/vitest.config.ts

@@ -0,0 +1,9 @@
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    environment: 'node',
+    include: ['src/**/*.test.ts'],
+    pool: 'forks',
+  },
+});