优化目录结构

backend/src/main/java/com/openisle/model/PushSubscription.java

@@ -0,0 +1,40 @@
+package com.openisle.model;
+
+import jakarta.persistence.*;
+import lombok.Getter;
+import lombok.NoArgsConstructor;
+import lombok.Setter;
+import org.hibernate.annotations.CreationTimestamp;
+
+import java.time.LocalDateTime;
+
+/**
+ * Entity storing a browser push subscription for a user.
+ */
+@Entity
+@Getter
+@Setter
+@NoArgsConstructor
+@Table(name = "push_subscriptions")
+public class PushSubscription {
+    @Id
+    @GeneratedValue(strategy = GenerationType.IDENTITY)
+    private Long id;
+
+    @ManyToOne(fetch = FetchType.LAZY, optional = false)
+    @JoinColumn(name = "user_id")
+    private User user;
+
+    @Column(nullable = false, length = 512)
+    private String endpoint;
+
+    @Column(nullable = false, length = 256)
+    private String p256dh;
+
+    @Column(nullable = false, length = 256)
+    private String auth;
+
+    @CreationTimestamp
+    @Column(nullable = false, updatable = false)
+    private LocalDateTime createdAt;
+}