Add subscription feature

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;
+}