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/tests/PnvPanel.Application.Tests/Admin/Pricing/UpdatePricingSettingsCommandValidatorTests.cs

@@ -0,0 +1,61 @@
+using PnvPanel.Application.Admin.Pricing;
+using PnvPanel.Application.Common.Interfaces;
+using Xunit;
+
+namespace PnvPanel.Application.Tests.Admin.Pricing;
+
+public class UpdatePricingSettingsCommandValidatorTests
+{
+    private readonly UpdatePricingSettingsCommandValidator _validator = new();
+
+    private static UpdatePricingSettingsCommand Command(params DiscountTierDto[] tiers) =>
+        new(500, 450, 400, tiers);
+
+    [Fact]
+    public void Validate_WithEmptyTiers_IsValid()
+    {
+        var result = _validator.Validate(Command());
+
+        Assert.True(result.IsValid);
+    }
+
+    [Fact]
+    public void Validate_WithProgressiveTiers_IsValid()
+    {
+        var result = _validator.Validate(Command(new DiscountTierDto(3, 5), new DiscountTierDto(6, 10)));
+
+        Assert.True(result.IsValid);
+    }
+
+    [Fact]
+    public void Validate_WithDuplicateThreshold_IsInvalid()
+    {
+        var result = _validator.Validate(Command(new DiscountTierDto(3, 5), new DiscountTierDto(3, 10)));
+
+        Assert.False(result.IsValid);
+    }
+
+    [Fact]
+    public void Validate_WhenHigherThresholdHasSmallerDiscount_IsInvalid()
+    {
+        var result = _validator.Validate(Command(new DiscountTierDto(3, 10), new DiscountTierDto(6, 5)));
+
+        Assert.False(result.IsValid);
+    }
+
+    [Fact]
+    public void Validate_WithDiscountPercentOutOfRange_IsInvalid()
+    {
+        var result = _validator.Validate(Command(new DiscountTierDto(3, 100)));
+
+        Assert.False(result.IsValid);
+    }
+
+    [Fact]
+    public void Validate_WithNonPositiveMinConfigs_IsInvalid()
+    {
+        var result = _validator.Validate(Command(new DiscountTierDto(0, 5)));
+
+        Assert.False(result.IsValid);
+    }
+}

backend/tests/PnvPanel.Application.Tests/Billing/CreatePaymentRequest/CreatePaymentRequestCommandHandlerTests.cs

@@ -65,6 +65,38 @@ public class CreatePaymentRequestCommandHandlerTests
         Assert.Equal(PaymentRequestStatus.AwaitingPayment, result.Value.Status);
     }
 
+    [Fact]
+    public async Task Handle_WithApplicableDiscountTier_AppliesDiscountToAmount()
+    {
+        using var dbContext = InMemoryDbContextFactory.Create();
+        var userId = Guid.NewGuid();
+        var pricing = PricingSettings.CreateDefault();
+        pricing.Update(500, 450, 400);
+        dbContext.PricingSettings.Add(pricing);
+        dbContext.PricingDiscountTiers.Add(PricingDiscountTier.Create(pricing.Id, minConfigs: 3, discountPercent: 10));
+        dbContext.PricingDiscountTiers.Add(PricingDiscountTier.Create(pricing.Id, minConfigs: 6, discountPercent: 20));
+        await dbContext.SaveChangesAsync(CancellationToken.None);
+
+        _identityService
+            .GetProfileAsync(userId, Arg.Any<CancellationToken>())
+            .Returns(Profile(userId, maxConfigs: 5));
+
+        var handler = new CreatePaymentRequestCommandHandler(
+            dbContext,
+            _identityService,
+            FakeCurrentUser.Authenticated(userId)
+        );
+
+        var result = await handler.Handle(
+            new CreatePaymentRequestCommand(PaymentPeriod.Quarter),
+            CancellationToken.None
+        );
+
+        Assert.True(result.IsSuccess);
+        // 500 * 5 * 3 = 7500, скидка 10% (порог 6 не достигнут при 5 конфигах) → 6750.
+        Assert.Equal(6750, result.Value.AmountSnapshot);
+    }
+
     [Fact]
     public async Task Handle_WhenBillingNotEnabled_ReturnsNotEnabled()
     {

backend/tests/PnvPanel.Domain.Tests/Pricing/PricingDiscountTests.cs

@@ -0,0 +1,64 @@
+using PnvPanel.Domain.Pricing;
+using Xunit;
+
+namespace PnvPanel.Domain.Tests.Pricing;
+
+public class PricingDiscountTests
+{
+    private static PricingDiscountTier Tier(int minConfigs, int percent) =>
+        PricingDiscountTier.Create(Guid.NewGuid(), minConfigs, percent);
+
+    [Fact]
+    public void ResolvePercent_WhenNoTiers_ReturnsZero()
+    {
+        var percent = PricingDiscount.ResolvePercent([], maxConfigs: 10);
+
+        Assert.Equal(0, percent);
+    }
+
+    [Fact]
+    public void ResolvePercent_WhenBelowAllThresholds_ReturnsZero()
+    {
+        var tiers = new[] { Tier(3, 5), Tier(6, 10) };
+
+        var percent = PricingDiscount.ResolvePercent(tiers, maxConfigs: 2);
+
+        Assert.Equal(0, percent);
+    }
+
+    [Fact]
+    public void ResolvePercent_PicksHighestApplicableTier_NotCumulative()
+    {
+        var tiers = new[] { Tier(3, 5), Tier(6, 10), Tier(12, 15) };
+
+        var percent = PricingDiscount.ResolvePercent(tiers, maxConfigs: 8);
+
+        Assert.Equal(10, percent);
+    }
+
+    [Fact]
+    public void ResolvePercent_ExactlyOnThreshold_Applies()
+    {
+        var tiers = new[] { Tier(3, 5) };
+
+        var percent = PricingDiscount.ResolvePercent(tiers, maxConfigs: 3);
+
+        Assert.Equal(5, percent);
+    }
+
+    [Fact]
+    public void Apply_WithZeroPercent_ReturnsAmountUnchanged()
+    {
+        var amount = PricingDiscount.Apply(1000, 0);
+
+        Assert.Equal(1000, amount);
+    }
+
+    [Fact]
+    public void Apply_WithPercent_RoundsToNearestInteger()
+    {
+        var amount = PricingDiscount.Apply(999, 10);
+
+        Assert.Equal(899, amount);
+    }
+}