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.Infrastructure/Persistence/AppDbContext.cs

@@ -59,6 +59,8 @@ public class AppDbContext(DbContextOptions<AppDbContext> options)
 
     public DbSet<PricingSettings> PricingSettings => Set<PricingSettings>();
 
+    public DbSet<PricingDiscountTier> PricingDiscountTiers => Set<PricingDiscountTier>();
+
     public DbSet<BillingSettings> BillingSettings => Set<BillingSettings>();
 
     public DbSet<PaymentRequest> PaymentRequests => Set<PaymentRequest>();