Backend Feature: AddToCart

User story

As a visitor, I want to add products to my cart so that I can select items before placing an order.

Dependencies

• Entities: Cart, CartItem, Product
• API specs: POST /cart/items

Acceptance criteria

• [ ] Creates a new open cart for the sessionId if one does not exist.
• [ ] Adds the product to the cart with the given quantity.
• [ ] If the product is already in the cart, sums the quantities.
• [ ] quantity must be at least 1; returns 400 otherwise.
• [ ] Product must exist and not be soft-deleted; returns 422 otherwise.
• [ ] Returns 200 with the updated cart contents.

Edge cases

• First item added: cart is created and item inserted.
• Same product added again: existing cart item quantity is incremented.
• Invalid productId: returns 422.
• quantity is 0 or negative: returns 400.

Notes

• The sessionId is always provided by the client — the API never generates it.