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/Plans/PlanErrors.cs

@@ -0,0 +1,26 @@
+using PnvPanel.Application.Common.Models;
+
+namespace PnvPanel.Application.Plans;
+
+public static class PlanErrors
+{
+    public static readonly Error NotFound = Error.NotFound("Plans.NotFound", "Тариф не найден.");
+
+    public static readonly Error Disabled = Error.Validation(
+        "Plans.Disabled",
+        "Тариф отключён."
+    );
+
+    public static Error MustSelectConfigsToRevoke(int requiredCount) =>
+        Error.Validation(
+            "Plans.MustSelectConfigsToRevoke",
+            $"Новый тариф меньше текущего количества активных конфигов — выберите {requiredCount} конфиг(ов) для отзыва."
+        );
+
+    /// <summary>Безлимитная квота (-1) зарезервирована за ролью admin — обычному пользователю
+    /// админ не может выставить безлимит напрямую через оверрайд тарифа.</summary>
+    public static readonly Error UnlimitedOnlyForAdmin = Error.Validation(
+        "Plans.UnlimitedOnlyForAdmin",
+        "Безлимитная квота конфигов доступна только для роли admin."
+    );
+}