Implement billing functionality and enhance role management

- Introduced billing capabilities, allowing users to request payments for subscription periods (3/6/12 months) with admin approval via Telegram.
- Updated role management to include a `BillingEnabled` property, preventing billing for admin roles.
- Enhanced the `CreateRoleCommand` and `UpdateRoleCommand` to accept billing parameters, ensuring proper handling during role creation and updates.
- Added new endpoints for billing management and integrated billing checks into VPN config creation to enforce payment requirements.
- Updated related services, models, and tests to support the new billing features, ensuring comprehensive coverage and functionality.
- Enhanced documentation to reflect the new billing processes and role management changes.

backend/src/PnvPanel.Application/Configs/Create/CreateVpnConfigCommandHandler.cs

@@ -36,6 +36,13 @@ public sealed class CreateVpnConfigCommandHandler(
         if (!inbound.AllowedRoleIds.Contains(profile.RoleId))
             return Result.Failure<VpnConfigDto>(ConfigErrors.InboundNotAllowedForRole);
 
+        // Не даём обойти приостановку за неуплату созданием нового конфига — см. BillingService.
+        if (
+            profile.BillingEnabled
+            && (profile.BillingPaidUntil is null || profile.BillingPaidUntil < DateTimeOffset.UtcNow)
+        )
+            return Result.Failure<VpnConfigDto>(ConfigErrors.BillingRequired);
+
         var node = await dbContext
             .Nodes.AsNoTracking()
             .FirstOrDefaultAsync(n => n.Id == inbound.NodeId, cancellationToken);
@@ -47,6 +54,8 @@ public sealed class CreateVpnConfigCommandHandler(
             return Result.Failure<VpnConfigDto>(ConfigErrors.NodeDisabled);
 
         var config = VpnConfig.Create(userId, inbound.Id, inbound.Protocol, command.Label);
+        if (profile.BillingEnabled)
+            config.SetBillingExpiry(profile.BillingPaidUntil);
 
         var reserveResult = await ReserveQuotaSlotAsync(
             userId,