Stage 9: Move a product rule into a service
JDK 21+; Maven 3.9+.
Open the folder containing pom.xml.
Run: mvn spring-boot:run
Open: http://localhost:8087/products
Stop: Ctrl+C
Complete explanations and all source: README.html
Expected: Compare /products (30 rows) and /products?inStockOnly=true (29 rows). Product 5 still has a details page with stock 0. An unknown ID returns 404.
