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/Support/CreateRoleRequest/CreateRoleRequestTicketCommandHandler.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;
@@ -7,6 +8,8 @@ using PnvPanel.Domain.Support;
 
 namespace PnvPanel.Application.Support.CreateRoleRequest;
 
+/// <summary>Проверка "нет висящей заявки" + создание — под AdvisoryLock (по UserId), см.
+/// CreatePaymentRequestCommandHandler для полного обоснования.</summary>
 public sealed class CreateRoleRequestTicketCommandHandler(
     IAppDbContext dbContext,
     IRoleService roleService,
@@ -27,47 +30,57 @@ public sealed class CreateRoleRequestTicketCommandHandler(
         if (currentUser.UserId is not { } userId)
             return Result.Failure<TicketDetailDto>(AuthErrors.Unauthorized);
 
-        var hasPending = await dbContext.SupportTickets.AnyAsync(
-            t =>
-                t.UserId == userId
-                && t.Type == TicketType.RoleRequest
-                && t.Status == TicketStatus.Open,
-            cancellationToken
-        );
-        if (hasPending)
-            return Result.Failure<TicketDetailDto>(SupportErrors.RoleRequestAlreadyPending);
-
-        SupportTicket ticket;
         string? requestedRoleName = null;
-
-        if (command.ExistingRoleId is { } roleId)
+        if (command.ExistingRoleId is { } existingRoleId)
         {
             var roles = await roleService.ListRolesAsync(cancellationToken);
-            var role = roles.FirstOrDefault(r => r.Id == roleId);
+            var role = roles.FirstOrDefault(r => r.Id == existingRoleId);
             if (role is null)
                 return Result.Failure<TicketDetailDto>(SupportErrors.RoleNotFound);
 
             if (role.Name.Equals(AdminRoleName, StringComparison.OrdinalIgnoreCase))
                 return Result.Failure<TicketDetailDto>(SupportErrors.CannotRequestAdminRole);
 
-            ticket = SupportTicket.CreateRoleRequestForExistingRole(userId, roleId);
             requestedRoleName = role.Name;
         }
-        else
-        {
-            ticket = SupportTicket.CreateRoleRequestForNewRole(
-                userId,
-                command.NewRoleName!,
-                command.NewRoleMaxConfigs!.Value,
-                command.NewRoleMaxIpLimit!.Value
-            );
-        }
 
-        dbContext.SupportTickets.Add(ticket);
+        var claimed = await AdvisoryLock.RunAsync(
+            dbContext,
+            userId,
+            async lockedCancellationToken =>
+            {
+                var hasPending = await dbContext.SupportTickets.AnyAsync(
+                    t =>
+                        t.UserId == userId
+                        && t.Type == TicketType.RoleRequest
+                        && t.Status == TicketStatus.Open,
+                    lockedCancellationToken
+                );
+                if (hasPending)
+                    return Result.Failure<(SupportTicket, TicketComment)>(
+                        SupportErrors.RoleRequestAlreadyPending
+                    );
 
-        var comment = TicketComment.Create(ticket.Id, userId, command.Justification);
-        dbContext.TicketComments.Add(comment);
+                var newTicket =
+                    command.ExistingRoleId is { } roleId
+                        ? SupportTicket.CreateRoleRequestForExistingRole(userId, roleId)
+                        : SupportTicket.CreateRoleRequestForNewRole(
+                            userId,
+                            command.NewRoleName!,
+                            command.NewRoleMaxConfigs!.Value,
+                            command.NewRoleMaxIpLimit!.Value
+                        );
+                dbContext.SupportTickets.Add(newTicket);
+                var newComment = TicketComment.Create(newTicket.Id, userId, command.Justification);
+                dbContext.TicketComments.Add(newComment);
+                return Result.Success((newTicket, newComment));
+            },
+            cancellationToken
+        );
+        if (!claimed.IsSuccess)
+            return Result.Failure<TicketDetailDto>(claimed.Error);
 
+        var (ticket, comment) = claimed.Value;
         var userName = currentUser.UserName ?? userId.ToString();
         var roleDescription =
             requestedRoleName