feat: 파일 저장 엔티티 추가

- ChatFile 엔티티 생성
- 파일 전송시 해당 데이터들을 관리

Resolves: #71

src/main/java/com/market/saessag/domain/chat/entity/ChatFile.java

@@ -0,0 +1,28 @@
+package com.market.saessag.domain.chat.entity;
+
+import jakarta.persistence.*;
+import lombok.AllArgsConstructor;
+import lombok.Builder;
+import lombok.Getter;
+import lombok.NoArgsConstructor;
+import org.hibernate.annotations.OnDelete;
+import org.hibernate.annotations.OnDeleteAction;
+
+@Entity
+@Getter
+@NoArgsConstructor
+@AllArgsConstructor
+@Builder
+public class ChatFile {
+    @Id
+    @GeneratedValue(strategy = GenerationType.IDENTITY)
+    private Long id;
+
+    @ManyToOne
+    @JoinColumn(name = "chat_message_id")
+    @OnDelete(action = OnDeleteAction.CASCADE)
+    private ChatMessage chatMessage;
+
+    private String fileUrl;
+    private String fileType;
+}

src/main/java/com/market/saessag/domain/chat/entity/ChatMessage.java

@@ -10,6 +10,8 @@
 import org.hibernate.annotations.OnDeleteAction;
 
 import java.time.LocalDateTime;
+import java.util.ArrayList;
+import java.util.List;
 
 @Entity
 @Getter
@@ -32,5 +34,8 @@ public class ChatMessage {
 
     private String content;
 
+    @OneToMany(mappedBy = "chatMessage", cascade = CascadeType.ALL, orphanRemoval = true)
+    private List<ChatFile> files = new ArrayList<>();
+
     private LocalDateTime timeStamp;
 }