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/Common/Interfaces/IAppDbContext.cs

@@ -49,6 +49,8 @@ public interface IAppDbContext
 
     DbSet<PricingSettings> PricingSettings { get; }
 
+    DbSet<PricingDiscountTier> PricingDiscountTiers { get; }
+
     DbSet<BillingSettings> BillingSettings { get; }
 
     DbSet<PaymentRequest> PaymentRequests { get; }

backend/src/PnvPanel.Application/Common/Interfaces/PricingSettingsDto.cs

@@ -2,19 +2,31 @@ using PnvPanel.Domain.Pricing;
 
 namespace PnvPanel.Application.Common.Interfaces;
 
-/// <summary>Все поля — цена за конфиг В МЕСЯЦ при соответствующем тарифе оплаты; итог за период =
-/// ставка × число месяцев. Используется и Admin (редактирование), и Support (справка при заявке на
-/// роль) — см. PricingSettingsDto.FromDomain.</summary>
+/// <summary>Одна ступень скидочной лесенки за объём — см. PricingDiscount.ResolvePercent для алгоритма
+/// выбора действующей ступени.</summary>
+public sealed record DiscountTierDto(int MinConfigs, int DiscountPercent);
+
+/// <summary>Все ценовые поля — цена за конфиг В МЕСЯЦ при соответствующем тарифе; итог за период =
+/// ставка × число месяцев, скидка (DiscountTiers) применяется к этому итогу по квоте роли. Используется
+/// и Admin (редактирование), и Support (справка при заявке на роль) — см. PricingSettingsDto.FromDomain.</summary>
 public sealed record PricingSettingsDto(
     int? PricePerConfigPerQuarter,
     int? PricePerConfigPerHalfYear,
-    int? PricePerConfigPerYear
+    int? PricePerConfigPerYear,
+    IReadOnlyList<DiscountTierDto> DiscountTiers
 )
 {
-    public static PricingSettingsDto FromDomain(PricingSettings settings) =>
+    public static PricingSettingsDto FromDomain(
+        PricingSettings settings,
+        IReadOnlyList<PricingDiscountTier> discountTiers
+    ) =>
         new(
             settings.PricePerConfigPerQuarter,
             settings.PricePerConfigPerHalfYear,
-            settings.PricePerConfigPerYear
+            settings.PricePerConfigPerYear,
+            discountTiers
+                .OrderBy(t => t.MinConfigs)
+                .Select(t => new DiscountTierDto(t.MinConfigs, t.DiscountPercent))
+                .ToList()
         );
 }