Build a Bookstore Inventory API

Problem/Feature Description

A small online bookstore needs a REST API to manage its inventory. Build it using Express and TypeScript.

Endpoints:

• GET /api/books -- list all books
• GET /api/books/:id -- get a single book by ID
• POST /api/books -- create a new book (fields: title, author, isbn, price)
• PUT /api/books/:id -- update a book
• DELETE /api/books/:id -- delete a book

Business rules:

• ISBN must be unique across all books
• Price must be a positive number
• Title and author are required non-empty strings

Use in-memory storage (no real database needed).

Output Specification

Produce:

• src/server.ts -- server entry point
• src/app.ts -- Express application setup
• src/routes/books.ts -- route handlers for book CRUD
• package.json -- with dependencies listed (no need to run npm install)

You may create additional files as needed for a well-structured codebase.