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/Migrations/AppDbContextModelSnapshot.cs

@@ -583,6 +583,29 @@ namespace PnvPanel.Infrastructure.Persistence.Migrations
                     b.ToTable("Nodes", (string)null);
                 });
 
+            modelBuilder.Entity("PnvPanel.Domain.Pricing.PricingDiscountTier", b =>
+                {
+                    b.Property<Guid>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("uuid");
+
+                    b.Property<int>("DiscountPercent")
+                        .HasColumnType("integer");
+
+                    b.Property<int>("MinConfigs")
+                        .HasColumnType("integer");
+
+                    b.Property<Guid>("PricingSettingsId")
+                        .HasColumnType("uuid");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("PricingSettingsId", "MinConfigs")
+                        .IsUnique();
+
+                    b.ToTable("PricingDiscountTiers", (string)null);
+                });
+
             modelBuilder.Entity("PnvPanel.Domain.Pricing.PricingSettings", b =>
                 {
                     b.Property<Guid>("Id")