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;
+}