feat: 添加推送通知功能，支持订阅和发送通知

Repository/PushSubscriptionRepository.cs

@@ -0,0 +1,16 @@
+namespace Repository;
+
+public interface IPushSubscriptionRepository : IBaseRepository<PushSubscriptionEntity>
+{
+    Task<PushSubscriptionEntity?> GetByEndpointAsync(string endpoint);
+}
+
+public class PushSubscriptionRepository(IFreeSql freeSql) : BaseRepository<PushSubscriptionEntity>(freeSql), IPushSubscriptionRepository
+{
+    public async Task<PushSubscriptionEntity?> GetByEndpointAsync(string endpoint)
+    {
+        return await FreeSql.Select<PushSubscriptionEntity>()
+            .Where(x => x.Endpoint == endpoint)
+            .FirstAsync();
+    }
+}