feat(backend): stats service and routes

packages/backend/src/app.ts

@@ -5,6 +5,7 @@ import { ApiError } from './lib/errors.js';
 import { cardsRouter } from './routes/cards.js';
 import { lessonsRouter } from './routes/lessons.js';
 import { sessionsRouter } from './routes/sessions.js';
+import { statsRouter } from './routes/stats.js';
 
 export function createApp(db: Db): Express {
   const app = express();
@@ -14,6 +15,7 @@ export function createApp(db: Db): Express {
   app.use('/api/lessons', lessonsRouter(db));
   app.use('/api', cardsRouter(db));
   app.use('/api/sessions', sessionsRouter(db));
+  app.use('/api/stats', statsRouter(db));
 
   app.use((err: unknown, _req: Request, res: Response, _next: NextFunction) => {
     if (err instanceof ZodError) {