Feat: Product 클래스에 재고 수량 차감 로직 구현

- 만약 주문 개수보다 재고 수량이 부족하다면 NotEnoughStockQuantityException이 발생한다.

BE/src/main/java/sidedish/com/domain/Product.java

@@ -2,6 +2,7 @@
 
 import java.util.List;
 import lombok.Getter;
+import sidedish.com.exception.NotEnoughStockQuantityException;
 
 @Getter
 public class Product {
@@ -16,7 +17,7 @@ public class Product {
 	private final String description;
 	private final long fixedPrice;
 	private final long originalPrice;
-	private final long stockQuantity;
+	private long stockQuantity;
 	private final String mealCategory;
 	private final String bestCategory;
 	private final long mileage;
@@ -25,6 +26,9 @@ public Product(Long id, DiscountPolicy discountPolicy,
 		DeliveryPolicy deliveryPolicy, List<Image> images, String productName,
 		String description, long originalPrice, long stockQuantity, String mealCategory,
 		String bestCategory) {
+		if (stockQuantity < 0) {
+			throw new NotEnoughStockQuantityException("재고가 충분하지 않습니다");
+		}
 		this.id = id;
 		this.discountPolicy = discountPolicy;
 		this.deliveryPolicy = deliveryPolicy;
@@ -46,4 +50,11 @@ private long calculateMileage() {
 	private long calculateFixedPrice() {
 		return discountPolicy.calculateFixedPrice(originalPrice);
 	}
+
+	public void minusStockQuantity(long count) {
+		if (count > stockQuantity) {
+			throw new NotEnoughStockQuantityException("재고가 충분하지 않습니다");
+		}
+		stockQuantity -= count;
+	}
 }

BE/src/main/java/sidedish/com/exception/NotEnoughStockQuantityException.java

@@ -0,0 +1,12 @@
+package sidedish.com.exception;
+
+import org.springframework.http.HttpStatus;
+import org.springframework.web.bind.annotation.ResponseStatus;
+
+@ResponseStatus(HttpStatus.BAD_REQUEST)
+public class NotEnoughStockQuantityException extends RuntimeException {
+
+    public NotEnoughStockQuantityException(String message) {
+        super(message);
+    }
+}