docs: 클래스다이어그램 추가

Author: yeonjiyeon

docs/week2/03-class-diagram.md

@@ -0,0 +1,82 @@
+## 클래스 다이어그램
+
+---
+```mermaid
+classDiagram
+    class User {
+        - Long id
+        - String userId
+        - String email
+        - LocalDate birthdate
+        - Gender gender
+        --
+        + boolean hasEnoughPoint(Long amount)
+        + void deductPoint(Long amount)
+    }
+
+    class Point {
+        - Long id
+        - User user
+        - Long value
+        --
+        + void increase(Long amount)
+        + void decrease(Long amount)
+    }
+
+    class Product {
+        - Long id
+        - Brand brand
+        - String name
+        - String description
+        - Long price
+        - int stock
+        --
+        + boolean isOutOfStock()
+        + boolean hasSufficientStock(int quantity)
+        + void decreaseStock(int quantity)
+    }
+
+    class Brand {
+        - Long id
+        - String name
+        - String description
+        --
+        + List~Product~ getProducts()
+    }
+
+    class Like {
+        - Long id
+        - User user
+        - Product product
+        - TIMESTAMP deleted_at
+        --
+        + void toggle()
+    }
+
+    class Order {
+        - Long id
+        - User user
+        - List~OrderItem~ items
+        - OrderStatus status
+        --
+        + Long getTotalPrice()
+        + void markPaid()
+    }
+
+    class OrderItem {
+        - Long id
+        - Product product
+        - int quantity
+        - Long price
+        --
+        + Long getSubTotal()
+    }
+
+    User "1" --> "1..*" Order
+    User "1" --> "1..*" Like
+    User "1" --> "1" Point
+    Order "1" --> "1..*" OrderItem
+    Product "1" --> "1..*" Like
+    Brand "1" --> "1..*" Product
+
+```