优化目录结构

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

@@ -0,0 +1,61 @@
+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;
+
+/**
+ * Entity representing a user notification.
+ */
+@Entity
+@Getter
+@Setter
+@NoArgsConstructor
+@Table(name = "notifications")
+public class Notification {
+    @Id
+    @GeneratedValue(strategy = GenerationType.IDENTITY)
+    private Long id;
+
+    @Enumerated(EnumType.STRING)
+    @Column(nullable = false)
+    private NotificationType type;
+
+    @ManyToOne(fetch = FetchType.LAZY, optional = false)
+    @JoinColumn(name = "user_id")
+    private User user;
+
+    @ManyToOne(fetch = FetchType.LAZY)
+    @JoinColumn(name = "post_id")
+    private Post post;
+
+    @ManyToOne(fetch = FetchType.LAZY)
+    @JoinColumn(name = "comment_id")
+    private Comment comment;
+
+    @ManyToOne(fetch = FetchType.LAZY)
+    @JoinColumn(name = "from_user_id")
+    private User fromUser;
+
+    @Enumerated(EnumType.STRING)
+    @Column(name = "reaction_type")
+    private ReactionType reactionType;
+
+    @Column(length = 1000)
+    private String content;
+
+    @Column
+    private Boolean approved;
+
+    @Column(name = "is_read", nullable = false)
+    private boolean read = false;
+
+    @CreationTimestamp
+    @Column(nullable = false, updatable = false,
+            columnDefinition = "DATETIME(6) DEFAULT CURRENT_TIMESTAMP(6)")
+    private LocalDateTime createdAt;
+}