Implement discount tiers for pricing settings and enhance related functionalities

- Introduced a new `DiscountTierDto` to represent volume discount tiers, allowing roles with a config quota at or above specified thresholds to receive discounts on pricing.
- Updated the `PricingSettingsDto` to include a list of discount tiers, enhancing the pricing model to support more flexible pricing strategies.
- Modified the `GetPricingSettingsQueryHandler` and `UpdatePricingSettingsCommandHandler` to handle discount tiers, ensuring they are correctly retrieved and updated in the database.
- Enhanced validation in `UpdatePricingSettingsCommandValidator` to enforce uniqueness and progressive discount tiers, preventing invalid configurations.
- Updated frontend components to support the new discount tier functionality, including forms for adding and managing discount tiers in the admin interface.
- Revised API documentation to reflect the new discount tier features and their usage in pricing settings.

backend/src/PnvPanel.Application/Support/GetSupportPricing/GetSupportPricingQueryHandler.cs

@@ -18,10 +18,14 @@ public sealed class GetSupportPricingQueryHandler(IAppDbContext dbContext)
             .FirstOrDefaultAsync(cancellationToken);
 
         // Ещё не сидировано/не сохранено ни разу — цена не задана, а не ошибка.
-        return Result.Success(
-            settings is null
-                ? new PricingSettingsDto(null, null, null)
-                : PricingSettingsDto.FromDomain(settings)
-        );
+        if (settings is null)
+            return Result.Success(new PricingSettingsDto(null, null, null, []));
+
+        var discountTiers = await dbContext
+            .PricingDiscountTiers.AsNoTracking()
+            .Where(t => t.PricingSettingsId == settings.Id)
+            .ToListAsync(cancellationToken);
+
+        return Result.Success(PricingSettingsDto.FromDomain(settings, discountTiers));
     }
 }