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.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);
+    }
+}