add: single_use products

crates/core/src/model/economy.rs

@@ -29,6 +29,8 @@ pub struct Product {
     ///
     /// A negative stock means the product has unlimited stock.
     pub stock: i32,
+    /// If this product is limited to one purchase per person.
+    pub single_use: bool,
 }
 
 impl Product {
@@ -44,6 +46,7 @@ impl Product {
             on_sale: false,
             price: 0,
             stock: 0,
+            single_use: true,
         }
     }
 }