refines MODELS around cart functionality, moves all pricing into Sku model, adds CouponCode model

MODELS.md

@@ -185,7 +185,6 @@ struct Track {
     is_available: bool,
     preview_source: Option<String>,
     source: String,
-    price: Option<rusty_money::Money>,
     created_by: PersonId,
     created_at: chrono::DateTime,
     modified_by: PersonId,
@@ -218,7 +217,6 @@ struct Album {
     is_available: bool,
     preview_source: Option<String>,
     source: String,
-    price: Option<rusty_money::Money>,
     created_by: PersonId,
     created_at: chrono::DateTime,
     modified_by: PersonId,
@@ -405,7 +403,6 @@ struct Collection {
     albums: Option<Vec<Album>>,
     tracks: Option<Vec<Track>>,
     other_products: Option<Vec<OtherProduct>>,
-    price: Option<rusty_money::Money>,
 }
 ```
 
@@ -429,7 +426,7 @@ struct OtherProduct {
     is_available: bool,
     deleted_by: Option<PersonId>,
     deleted_at: Option<chrono::DateTime>,
-    price: Option<rusty_money::Money>
+    requires_shipping: bool,
 }
 ```
 
@@ -457,8 +454,9 @@ struct Sku {
     collection_id: Option<CollectionId>,
     other_product_id: Option<OtherProductId>,
     variant: Option<String>,
-    discount_percentage: Option<f64>,
-    discount_flatrate: Option<rusty_money::Money>,
+    description: Option<String>,
+    price: Option<rusty_money::Money>,
+    requires_shipping: bool,
 }
 struct PurchaseItem {
     id: PurchaseItemId,
@@ -481,5 +479,35 @@ struct Purchase {
     state: PurchaseState,
     purchased_by: Person,
     purchased_at: Option<chrono::DateTime>,
+    fulfilled_by: Option<Person>,
+    fulfilled_at: Option<chrono::DateTime>,
+}
+```
+
+# CouponCode
+
+A CouponCode represents a code a Person can enter to receive a discount on a PurchateItem or on a Purchase.
+
+``` rust
+struct CouponCodeId {
+    id: String,
+}
+struct CouponCode {
+    id: CouponCodeId,
+    name: String,
+    code: String,
+    uses: usize,
+    max_uses: usize,
+    expiration: Option<chrono::DateTime>,
+    discount_flat: Option<rusty_money::Money>,
+    discount_percentage: Option<f32>,
+    skus: Option<Vec<SkuId>>,
+    is_active: bool,
+    created_by: PersonId,
+    created_at: chrono::DateTime,
+    modified_by: PersonId,
+    modified_at: chrono::DateTime,
+    deleted_by: Option<PersonId>,
+    deleted_at: Option<chrono::DateTime>,
 }
 ```