Stock levels are initialized and tracked accurately

A product schema without a stock field, or with a nullable stock that defaults to null, means the application has no authoritative source for inventory state. Every in-stock/out-of-stock display becomes a guess, overselling becomes possible on the first order, and inventory reports return NULL aggregates that silently break dashboards. ISO 25010:2011 functional-correctness requires correct outputs; a stock value of null produces neither "in stock" nor "out of stock" — it produces an error or a wrong assumption depending on how null is coerced in the display layer. Tracking stock is not optional for e-commerce.

Add a stock field with a default of 0 in prisma/schema.prisma so newly created products start tracked rather than untracked.

model Product {
  id    String @id @default(cuid())
  name  String
  stock Int    @default(0)
}

In src/app/api/products/route.ts, always pass stock explicitly on creation rather than relying on the default — this prevents silent omissions in bulk import paths:

const product = await prisma.product.create({
  data: { name, price, stock: initialStock ?? 0 },
})