Enhance user plan management and update related endpoints

- Added new configuration options for user plans in `.env.example`, including `Plans__MaxCustomConfigCount` and `Plans__MinCustomConfigCount`.
- Introduced `MapPlanEndpoints` in `Program.cs` to handle plan-related API routes.
- Implemented `SetUserPlan` endpoint in `RoleEndpoints` to allow admins to assign plans to users.
- Removed deprecated role request approval endpoints from `AdminSupportEndpoints`.
- Updated `ITelegramNotifier` and related classes to reflect changes in role request handling and payment notifications.
- Refactored role management commands to remove `MaxConfigs` and focus on `MaxIpLimit` and billing settings.
- Enhanced billing request handling to accommodate plan changes instead of role changes.
- Updated various interfaces and command handlers to support new plan management features.

backend/src/PnvPanel.Application/Admin/Users/AdminSetUserPlanCommandValidator.cs

@@ -0,0 +1,18 @@
+using FluentValidation;
+
+namespace PnvPanel.Application.Admin.Users;
+
+public sealed class AdminSetUserPlanCommandValidator : AbstractValidator<AdminSetUserPlanCommand>
+{
+    public AdminSetUserPlanCommandValidator()
+    {
+        RuleFor(x => x)
+            .Must(x => x.PlanId.HasValue ^ x.CustomConfigCount.HasValue)
+            .WithMessage("Укажите либо PlanId, либо CustomConfigCount — ровно одно из двух.");
+
+        When(
+            x => x.CustomConfigCount.HasValue,
+            () => RuleFor(x => x.CustomConfigCount!.Value).GreaterThanOrEqualTo(-1)
+        );
+    }
+}