MS-311/feat(cart): Add shippingPrice and shippingPriceAfterDiscount fields to Cart entity

Summary

Adds two new persistent fields to the Cart entity to support shipping cost tracking and discount calculations at the cart level.

Changes

Cart.java

• Added shippingPrice (double) — stores the base shipping cost for the cart
• Added shippingPriceAfterDiscount (double) — stores the shipping cost after any applicable discount is applied

Database Impact

Two new columns will be added to the cart table:

• shipping_price
• shipping_price_after_discount

Linked Issue

Closes #MS-311