feat(backend): lessons CRUD service and routes

packages/backend/src/app.ts

@@ -1,28 +1,23 @@
 import express, { type Express, type NextFunction, type Request, type Response } from 'express';
 import { ZodError } from 'zod';
+import type { Db } from './db/client.js';
 import { ApiError } from './lib/errors.js';
+import { lessonsRouter } from './routes/lessons.js';
 
-export function createApp(): Express {
+export function createApp(db: Db): 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.get('/api/health', (_req, res) => res.json({ ok: true }));
+  app.use('/api/lessons', lessonsRouter(db));
 
   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() },
-      });
+      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 },
-      });
+      res.status(err.status).json({ error: { code: err.code, message: err.message, details: err.details } });
       return;
     }
     console.error(err);