feat(basket): add basket items entity and repository

[ainhprzz]

What

Adds support for storing products inside a basket by introducing BasketItem.

Changes

• Added BasketItem entity (basket, product, quantity)
• Added BasketItemRepository (findByBasket, findByBasketAndProduct)
• Updated Basket entity with OneToMany(items)
• (Optional) Event handler to update basket timestamps when items change

API / Data

• New resource: /basketItems (Spring Data REST)
• Basket now exposes items relation (HAL links)

Testing

1. Run backend
2. POST /basketItems with body:
   { "basket": "/baskets/{id}", "product": "/products/{id}", "quantity": 1 }
3. GET /baskets/{id} and verify items/links

Notes

Frontend PR depends on this PR.