feat(cart): update CartItem model to use composite key and adjust related logic

fix(cart): ensure cart item updates and deletions are based on cartId and itemId

feat(buy): implement balance checks and updates during purchase process

fix(sellable): ensure cart items are deleted when a sellable is removed

feat(user): add conflict checks for shop creation to prevent ID collisions

prisma/schema.prisma

@@ -117,12 +117,14 @@ model Cart {
 }
 
 model CartItem {
-  itemId   String @id
+  itemId   String
   quantity Int
   cartId   String
 
   cart     Cart     @relation(fields: [cartId], references: [userId], onDelete: Cascade, onUpdate: Cascade)
   sellable Sellable @relation(fields: [itemId], references: [id], onDelete: Cascade, onUpdate: Cascade)
+
+  @@id([cartId, itemId])
 }
 
 //////////////////////