优化目录结构

backend/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;
+}