feat: add notification system

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

@@ -0,0 +1,52 @@
+package com.openisle.model;
+
+import jakarta.persistence.*;
+import lombok.Getter;
+import lombok.NoArgsConstructor;
+import lombok.Setter;
+
+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;
+
+    @Column
+    private Boolean approved;
+
+    @Column(nullable = false)
+    private boolean read = false;
+
+    @Column(nullable = false)
+    private LocalDateTime createdAt;
+
+    @PrePersist
+    protected void onCreate() {
+        this.createdAt = LocalDateTime.now();
+    }
+}

src/main/java/com/openisle/model/NotificationType.java

@@ -0,0 +1,15 @@
+package com.openisle.model;
+
+/**
+ * Types of user notifications.
+ */
+public enum NotificationType {
+    /** Someone viewed your post */
+    POST_VIEWED,
+    /** Someone replied to your post or comment */
+    COMMENT_REPLY,
+    /** Someone reacted to your post or comment */
+    REACTION,
+    /** Your post under review was approved or rejected */
+    POST_REVIEWED
+}

src/main/java/com/openisle/model/PostStatus.java

@@ -5,5 +5,6 @@ package com.openisle.model;
  */
 public enum PostStatus {
     PUBLISHED,
-    PENDING
+    PENDING,
+    REJECTED
 }