Add subscription feature

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

@@ -0,0 +1,27 @@
+package com.openisle.model;
+
+import jakarta.persistence.*;
+import lombok.Getter;
+import lombok.NoArgsConstructor;
+import lombok.Setter;
+
+/** Subscription to a comment for replies notifications. */
+@Entity
+@Getter
+@Setter
+@NoArgsConstructor
+@Table(name = "comment_subscriptions",
+       uniqueConstraints = @UniqueConstraint(columnNames = {"user_id", "comment_id"}))
+public class CommentSubscription {
+    @Id
+    @GeneratedValue(strategy = GenerationType.IDENTITY)
+    private Long id;
+
+    @ManyToOne(fetch = FetchType.LAZY, optional = false)
+    @JoinColumn(name = "user_id")
+    private User user;
+
+    @ManyToOne(fetch = FetchType.LAZY, optional = false)
+    @JoinColumn(name = "comment_id")
+    private Comment comment;
+}

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

@@ -11,5 +11,9 @@ public enum NotificationType {
     /** Someone reacted to your post or comment */
     REACTION,
     /** Your post under review was approved or rejected */
-    POST_REVIEWED
+    POST_REVIEWED,
+    /** A subscribed post received a new comment */
+    POST_UPDATED,
+    /** A user you subscribe to created a post or comment */
+    USER_ACTIVITY
 }

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

@@ -0,0 +1,27 @@
+package com.openisle.model;
+
+import jakarta.persistence.*;
+import lombok.Getter;
+import lombok.NoArgsConstructor;
+import lombok.Setter;
+
+/** Subscription to a post for update notifications. */
+@Entity
+@Getter
+@Setter
+@NoArgsConstructor
+@Table(name = "post_subscriptions",
+       uniqueConstraints = @UniqueConstraint(columnNames = {"user_id", "post_id"}))
+public class PostSubscription {
+    @Id
+    @GeneratedValue(strategy = GenerationType.IDENTITY)
+    private Long id;
+
+    @ManyToOne(fetch = FetchType.LAZY, optional = false)
+    @JoinColumn(name = "user_id")
+    private User user;
+
+    @ManyToOne(fetch = FetchType.LAZY, optional = false)
+    @JoinColumn(name = "post_id")
+    private Post post;
+}

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

@@ -0,0 +1,27 @@
+package com.openisle.model;
+
+import jakarta.persistence.*;
+import lombok.Getter;
+import lombok.NoArgsConstructor;
+import lombok.Setter;
+
+/** Following relationship between users. */
+@Entity
+@Getter
+@Setter
+@NoArgsConstructor
+@Table(name = "user_subscriptions",
+       uniqueConstraints = @UniqueConstraint(columnNames = {"subscriber_id", "target_id"}))
+public class UserSubscription {
+    @Id
+    @GeneratedValue(strategy = GenerationType.IDENTITY)
+    private Long id;
+
+    @ManyToOne(fetch = FetchType.LAZY, optional = false)
+    @JoinColumn(name = "subscriber_id")
+    private User subscriber;
+
+    @ManyToOne(fetch = FetchType.LAZY, optional = false)
+    @JoinColumn(name = "target_id")
+    private User target;
+}