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>();

backend/src/PnvPanel.Infrastructure/Persistence/Configurations/PricingDiscountTierConfiguration.cs

@@ -0,0 +1,16 @@
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Metadata.Builders;
+using PnvPanel.Domain.Pricing;
+
+namespace PnvPanel.Infrastructure.Persistence.Configurations;
+
+public class PricingDiscountTierConfiguration : IEntityTypeConfiguration<PricingDiscountTier>
+{
+    public void Configure(EntityTypeBuilder<PricingDiscountTier> builder)
+    {
+        builder.ToTable("PricingDiscountTiers");
+        builder.HasKey(x => x.Id);
+
+        builder.HasIndex(x => new { x.PricingSettingsId, x.MinConfigs }).IsUnique();
+    }
+}

backend/src/PnvPanel.Infrastructure/Persistence/Migrations/20260719124434_AddPricingDiscountTiers.cs

@@ -0,0 +1,42 @@
+using System;
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace PnvPanel.Infrastructure.Persistence.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddPricingDiscountTiers : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.CreateTable(
+                name: "PricingDiscountTiers",
+                columns: table => new
+                {
+                    Id = table.Column<Guid>(type: "uuid", nullable: false),
+                    PricingSettingsId = table.Column<Guid>(type: "uuid", nullable: false),
+                    MinConfigs = table.Column<int>(type: "integer", nullable: false),
+                    DiscountPercent = table.Column<int>(type: "integer", nullable: false)
+                },
+                constraints: table =>
+                {
+                    table.PrimaryKey("PK_PricingDiscountTiers", x => x.Id);
+                });
+
+            migrationBuilder.CreateIndex(
+                name: "IX_PricingDiscountTiers_PricingSettingsId_MinConfigs",
+                table: "PricingDiscountTiers",
+                columns: new[] { "PricingSettingsId", "MinConfigs" },
+                unique: true);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropTable(
+                name: "PricingDiscountTiers");
+        }
+    }
+}

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")