e-commerce schemas questions

I was looking at the schema for users and noticed some things that can be updated:

### Cart:

```
type Cart {
"""
Items saved in the cart session
"""
items: [Product]

"""
The current total of all the items in the cart, before taxes and shipping
"""
subtotal: Float
}
```

I think we can change this to

```
type Cart {
"""
Items saved in the cart session
"""
items: [Product!]! # made this non nullable

"""
The current total of all the items in the cart, before taxes and shipping
"""
subtotal: Money! # made this money and non nullable (default could be 0 USD?)
}
```

### OrderFilters

```
input OrderFilters {
orderId: ID!
priceHigh: Float
priceLow: Float
itemsInOrder: Int
}
```

is `orderId` non-nullable on purpose?

---

There's other places where we have nullable list of nullables, I can update those to non-nullable list of non-nullables if we prefer that approach :)

### user and orders

Users shouldn't have orders inside this subgraph

```
"""
An user account in our system
"""
type User @key(fields: "id") {
id: ID!

"""
The users login username
"""
username: String!

"""
The user's active cart session. Once the cart items have been purchases, they transition to an Order
"""
cart: Cart

"""
The users previous purchases
"""
orders(filters: OrderFilters): [Order]
```

The issue names GraphQL types Cart, OrderFilters, User, and Order but provides no file paths or tests. Start by locating these schema definitions, then resolve the nullability, Money, and User/orders ownership questions with maintainers. Done means the agreed schema changes are applied and validated by the project's available checks.

Written by the indexing model from the issue text.