feat: 添加推送通知功能，支持订阅和发送通知

Entity/PushSubscriptionEntity.cs

@@ -0,0 +1,17 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace Entity;
+
+public class PushSubscriptionEntity : BaseEntity
+{
+    [Required]
+    public string Endpoint { get; set; } = string.Empty;
+
+    public string? P256DH { get; set; }
+
+    public string? Auth { get; set; }
+
+    public string? UserId { get; set; } // Optional: if you have user authentication
+    
+    public string? UserAgent { get; set; }
+}