Implement billing status notification and enhance user management integration

- Added `NotifyBillingStatusChangedAsync` method to `IRealtimeNotifier` for notifying clients about changes in billing status.
- Updated `BillingConfigResumer` to call the new notification method after modifying billing configurations, ensuring users receive real-time updates.
- Enhanced `ListUsersQueryHandler` to include a `BillingPendingReview` property in `UserSummaryDto`, indicating if a user has a pending payment request awaiting confirmation.
- Refactored various command handlers to utilize `AdvisoryLock` for managing concurrent requests, preventing race conditions in billing operations.
- Updated tests to cover new notification behaviors and ensure proper functionality in billing status management.

backend/tests/PnvPanel.Application.Tests/Admin/Users/ListUsersQueryHandlerTests.cs

@@ -0,0 +1,81 @@
+using NSubstitute;
+using PnvPanel.Application.Admin.Users;
+using PnvPanel.Application.Common.Interfaces;
+using PnvPanel.Application.Common.Models;
+using PnvPanel.Application.Tests.TestSupport;
+using PnvPanel.Domain.Billing;
+using Xunit;
+
+namespace PnvPanel.Application.Tests.Admin.Users;
+
+public class ListUsersQueryHandlerTests
+{
+    private readonly IIdentityService _identityService = Substitute.For<IIdentityService>();
+
+    private static UserSummaryDto Summary(Guid id) =>
+        new(id, "alice", "premium", true, false, DateTimeOffset.UtcNow, true, DateTimeOffset.UtcNow.AddDays(-1));
+
+    [Fact]
+    public async Task Handle_WhenUserHasAwaitingConfirmationSubscriptionRequest_SetsBillingPendingReview()
+    {
+        using var dbContext = InMemoryDbContextFactory.Create();
+        var userId = Guid.NewGuid();
+        var request = PaymentRequest.Create(userId, PaymentPeriod.Quarter, 1000);
+        request.MarkPaymentSent();
+        dbContext.PaymentRequests.Add(request);
+        await dbContext.SaveChangesAsync(CancellationToken.None);
+
+        _identityService
+            .ListUsersAsync(1, 20, null, Arg.Any<CancellationToken>())
+            .Returns(new PagedList<UserSummaryDto>([Summary(userId)], 1, 1, 20));
+
+        var handler = new ListUsersQueryHandler(_identityService, dbContext);
+
+        var result = await handler.Handle(new ListUsersQuery(1, 20, null), CancellationToken.None);
+
+        Assert.True(result.IsSuccess);
+        Assert.True(result.Value.Items.Single().BillingPendingReview);
+    }
+
+    [Fact]
+    public async Task Handle_WhenNoPendingRequest_LeavesBillingPendingReviewFalse()
+    {
+        using var dbContext = InMemoryDbContextFactory.Create();
+        var userId = Guid.NewGuid();
+
+        _identityService
+            .ListUsersAsync(1, 20, null, Arg.Any<CancellationToken>())
+            .Returns(new PagedList<UserSummaryDto>([Summary(userId)], 1, 1, 20));
+
+        var handler = new ListUsersQueryHandler(_identityService, dbContext);
+
+        var result = await handler.Handle(new ListUsersQuery(1, 20, null), CancellationToken.None);
+
+        Assert.True(result.IsSuccess);
+        Assert.False(result.Value.Items.Single().BillingPendingReview);
+    }
+
+    [Fact]
+    public async Task Handle_WhenPendingRequestIsRoleChangeTopUp_LeavesBillingPendingReviewFalse()
+    {
+        // RoleChangeTopUp — доплата за смену роли, не подписка; не должна показывать пользователя
+        // как "оплата на проверке" в админке (см. CLAUDE.md про Kind).
+        using var dbContext = InMemoryDbContextFactory.Create();
+        var userId = Guid.NewGuid();
+        var request = PaymentRequest.CreateRoleChangeTopUp(userId, 500);
+        request.MarkPaymentSent();
+        dbContext.PaymentRequests.Add(request);
+        await dbContext.SaveChangesAsync(CancellationToken.None);
+
+        _identityService
+            .ListUsersAsync(1, 20, null, Arg.Any<CancellationToken>())
+            .Returns(new PagedList<UserSummaryDto>([Summary(userId)], 1, 1, 20));
+
+        var handler = new ListUsersQueryHandler(_identityService, dbContext);
+
+        var result = await handler.Handle(new ListUsersQuery(1, 20, null), CancellationToken.None);
+
+        Assert.True(result.IsSuccess);
+        Assert.False(result.Value.Items.Single().BillingPendingReview);
+    }
+}