feat:【站内信】

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

@@ -0,0 +1,30 @@
+package com.openisle.model;
+
+import jakarta.persistence.*;
+import lombok.Getter;
+import lombok.NoArgsConstructor;
+import lombok.Setter;
+
+import java.time.LocalDateTime;
+
+@Entity
+@Getter
+@Setter
+@NoArgsConstructor
+@Table(name = "message_participants")
+public class MessageParticipant {
+    @Id
+    @GeneratedValue(strategy = GenerationType.IDENTITY)
+    private Long id;
+
+    @ManyToOne(optional = false, fetch = FetchType.LAZY)
+    @JoinColumn(name = "conversation_id")
+    private MessageConversation conversation;
+
+    @ManyToOne(optional = false, fetch = FetchType.LAZY)
+    @JoinColumn(name = "user_id")
+    private User user;
+
+    @Column
+    private LocalDateTime lastReadAt;
+}