feat:【站内信】

backend/src/main/java/com/openisle/model/MessageConversation.java

@@ -0,0 +1,36 @@
+package com.openisle.model;
+
+import jakarta.persistence.*;
+import lombok.Getter;
+import lombok.NoArgsConstructor;
+import lombok.Setter;
+import org.hibernate.annotations.CreationTimestamp;
+
+import java.time.LocalDateTime;
+import java.util.HashSet;
+import java.util.Set;
+
+@Entity
+@Getter
+@Setter
+@NoArgsConstructor
+@Table(name = "message_conversations")
+public class MessageConversation {
+    @Id
+    @GeneratedValue(strategy = GenerationType.IDENTITY)
+    private Long id;
+
+    @CreationTimestamp
+    @Column(nullable = false, updatable = false)
+    private LocalDateTime createdAt;
+
+    @OneToOne(fetch = FetchType.LAZY)
+    @JoinColumn(name = "last_message_id")
+    private Message lastMessage;
+
+    @OneToMany(mappedBy = "conversation", cascade = CascadeType.ALL, orphanRemoval = true)
+    private Set<MessageParticipant> participants = new HashSet<>();
+
+    @OneToMany(mappedBy = "conversation", cascade = CascadeType.ALL, orphanRemoval = true)
+    private Set<Message> messages = new HashSet<>();
+}