docs: 클래스 다이어그램 작성

Author: Kimjipang

docs/design/03-class-diagram.md

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+# 클래스 다이어그램
+
+```mermaid
+classDiagram
+	class Users {
+		- Long id
+		- String loginId
+		- String email
+		- Gender gender
+		- String password
+		
+		+ void changePassword(String password)
+		+ void changeEmail(String email)
+	}
+	
+	class Point {
+		- Long id
+		- Long refUserId
+		- int balance
+		
+		+ void chargePoint(int amount)
+		+ void usePoint(int amount)	
+	}
+	
+	class Brand {
+    - Long id
+    - String name
+    
+  }
+	
+  class Product {
+    - Long id
+    - Long refBrandId
+    - String name
+    - BigDecimal price
+    - int stock
+    
+    + void changePrice(BigDecimal newPrice)
+    + void discountPrice(BigDecimal rate)
+    + void increaseStock(int amount)
+    + void decreaseStock(int amount)
+    + boolean availableStock(int amount)
+    
+  }
+  
+  class Like {
+	  - Long id
+	  - Long refUserId
+		- Long refProductId
+		
+		+ static void createLike(Long userId, Long productId)
+		
+  }
+  
+	class Cart {
+	  - Long id
+	  - Long refUserId
+	  
+	  + void addItemToCart(Long productId, int amount)
+	  + void deleteItemFromCart(Long productId, int amount)
+  }
+
+  class CartItem {
+	  - Long id
+	  - String name
+	  - Long refCartId
+	  - Long refProductId
+		- int quantity
+		
+		+ void changeQuantity(int amount)
+  }
+  
+  class Order {
+	  - Long id
+	  - Long refUserId
+	  - OrderStatus status
+	  
+	  + void addItemToOrder(Product product, int quantity)
+	  + BigDecimal calculateTotalPrice()
+	  + void complete()
+	  + void cancel()
+	  
+  }
+  
+  class OrderItem {
+	  - Long id
+	  - Long refOrderId
+	  - Long refProductId
+	  - Integer quantity
+	  - BigDecimal orderPrice
+  }
+  
+  class Payment {
+	  - Long id
+	  - Long refOrderId
+	  - BigDecimal totalPrice
+	  - PaymentStatus status
+	  - PaymentMethod method
+	  
+	  + void requestPayment(Order order)
+	  + void completePayment()
+	  + void cancelPayment()
+	  + boolean isCompleted()
+	  + boolean isRefundable()
+  }
+  
+  Order "1" *-- "0..*" OrderItem
+  Cart "1" *-- "0..*" CartItem
+  Product "0..*" --> "1" Brand
+  Like "0..*" --> "1" Users
+  Like "0..*" --> "1" Product
+  Order "0..*" --> "1" Users
+  Point "1" --> "1" Users
+  Payment "1" --> "1" Order
+```