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/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()
     {