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);
+    }
+}