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/Billing/CreatePaymentRequest/CreatePaymentRequestCommandHandler.cs

@@ -4,6 +4,7 @@ using PnvPanel.Application.Common.Interfaces;
 using PnvPanel.Application.Common.Messaging;
 using PnvPanel.Application.Common.Models;
 using PnvPanel.Domain.Billing;
+using PnvPanel.Domain.Pricing;
 
 namespace PnvPanel.Application.Billing.CreatePaymentRequest;
 
@@ -44,17 +45,26 @@ public sealed class CreatePaymentRequestCommandHandler(
             return Result.Failure<PaymentRequestDto>(BillingErrors.ActiveRequestExists);
 
         var pricing = await dbContext.PricingSettings.AsNoTracking().FirstOrDefaultAsync(cancellationToken);
+        if (pricing is null)
+            return Result.Failure<PaymentRequestDto>(BillingErrors.PricingNotConfigured);
+
         var ratePerMonth = command.Period switch
         {
-            PaymentPeriod.Quarter => pricing?.PricePerConfigPerQuarter,
-            PaymentPeriod.HalfYear => pricing?.PricePerConfigPerHalfYear,
-            PaymentPeriod.Year => pricing?.PricePerConfigPerYear,
+            PaymentPeriod.Quarter => pricing.PricePerConfigPerQuarter,
+            PaymentPeriod.HalfYear => pricing.PricePerConfigPerHalfYear,
+            PaymentPeriod.Year => pricing.PricePerConfigPerYear,
             _ => null,
         };
         if (ratePerMonth is not { } rate)
             return Result.Failure<PaymentRequestDto>(BillingErrors.PricingNotConfigured);
 
-        var amount = rate * profile.MaxConfigs * command.Period.ToMonths();
+        var discountTiers = await dbContext
+            .PricingDiscountTiers.AsNoTracking()
+            .Where(t => t.PricingSettingsId == pricing.Id)
+            .ToListAsync(cancellationToken);
+        var discountPercent = PricingDiscount.ResolvePercent(discountTiers, profile.MaxConfigs);
+
+        var amount = PricingDiscount.Apply(rate * profile.MaxConfigs * command.Period.ToMonths(), discountPercent);
 
         var request = PaymentRequest.Create(userId, command.Period, amount);
         dbContext.PaymentRequests.Add(request);