Bookstore Inventory Service

Problem/Feature Description

A local bookstore wants a Flask API to manage their inventory. They currently track everything in spreadsheets and want to move to a simple web service. The API should manage books (title, author, isbn, price, quantity_in_stock) and categories (name, description). A book belongs to one category.

Requirements:

1. CRUD endpoints for books and categories.
2. A GET /api/books?category=fiction&in_stock=true endpoint with optional filtering.
3. A POST /api/books/<id>/restock endpoint that increases a book's quantity.
4. Rate limiting on write endpoints (create, update, restock) to prevent accidental duplicate submissions.
5. Proper validation -- ISBN must be provided, price must be positive, quantity can't be negative.
6. Use SQLite for persistence with a simple schema.

The bookstore owner is not technical, so error messages should be clear and the API should never show Python tracebacks.

Output Specification

Produce Python source files in a bookstore/ directory:

• app/__init__.py -- application factory
• app/extensions.py -- CORS, rate limiter instances
• app/errors.py -- custom exceptions and error handlers
• app/db.py -- SQLite setup, init_db(), get_db()
• app/routes/books.py -- books blueprint
• app/routes/categories.py -- categories blueprint
• tests/conftest.py -- test fixtures with in-memory SQLite
• run.py -- entry point
• requirements.txt

Do not run pip install or start the server.