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/src/PnvPanel.Application/Admin/Support/RejectRoleRequestCommandHandler.cs

@@ -1,5 +1,6 @@
 using Microsoft.EntityFrameworkCore;
 using PnvPanel.Application.Auth;
+using PnvPanel.Application.Common.Concurrency;
 using PnvPanel.Application.Common.Interfaces;
 using PnvPanel.Application.Common.Messaging;
 using PnvPanel.Application.Common.Models;
@@ -9,6 +10,8 @@ using PnvPanel.Domain.Support;
 
 namespace PnvPanel.Application.Admin.Support;
 
+/// <summary>Проверка статуса + Close() — под AdvisoryLock (по Id тикета), см.
+/// ApproveRoleRequestCommandHandler для полного обоснования.</summary>
 public sealed class RejectRoleRequestCommandHandler(
     IAppDbContext dbContext,
     IRealtimeNotifier notifier,
@@ -24,23 +27,36 @@ public sealed class RejectRoleRequestCommandHandler(
         if (currentUser.UserId is not { } adminId)
             return Result.Failure(AuthErrors.Unauthorized);
 
-        var ticket = await dbContext.SupportTickets.FirstOrDefaultAsync(
-            t => t.Id == command.TicketId,
+        var claimed = await AdvisoryLock.RunAsync(
+            dbContext,
+            command.TicketId,
+            async lockedCancellationToken =>
+            {
+                var fresh = await dbContext.SupportTickets.FirstOrDefaultAsync(
+                    t => t.Id == command.TicketId,
+                    lockedCancellationToken
+                );
+                if (fresh is null)
+                    return Result.Failure<SupportTicket>(SupportErrors.NotFound);
+
+                if (fresh.Type != TicketType.RoleRequest)
+                    return Result.Failure<SupportTicket>(SupportErrors.NotRoleRequest);
+
+                if (fresh.Status == TicketStatus.Closed)
+                    return Result.Failure<SupportTicket>(SupportErrors.AlreadyClosed);
+
+                if (!string.IsNullOrWhiteSpace(command.Reason))
+                    dbContext.TicketComments.Add(TicketComment.Create(fresh.Id, adminId, command.Reason));
+
+                fresh.Close();
+                return Result.Success(fresh);
+            },
             cancellationToken
         );
-        if (ticket is null)
-            return Result.Failure(SupportErrors.NotFound);
+        if (!claimed.IsSuccess)
+            return Result.Failure(claimed.Error);
 
-        if (ticket.Type != TicketType.RoleRequest)
-            return Result.Failure(SupportErrors.NotRoleRequest);
-
-        if (ticket.Status == TicketStatus.Closed)
-            return Result.Failure(SupportErrors.AlreadyClosed);
-
-        if (!string.IsNullOrWhiteSpace(command.Reason))
-            dbContext.TicketComments.Add(TicketComment.Create(ticket.Id, adminId, command.Reason));
-
-        ticket.Close();
+        var ticket = claimed.Value;
 
         dbContext.AuditLogs.Add(
             AuditLog.Create(