Update pricing model to include half-year pricing and enhance validation

- Introduced a new pricing field, `PricePerConfigPerHalfYear`, to the `PricingSettings` model, allowing for more flexible pricing options.
- Updated the `UpdatePricingSettingsCommand` and its validator to include the new half-year pricing, ensuring proper validation against the quarterly and yearly rates.
- Modified the `PricingSettingsDto` and related frontend components to accommodate the new half-year pricing field, including validation logic to prevent pricing discrepancies.
- Enhanced API documentation and frontend forms to reflect the updated pricing structure and validation rules.

backend/src/PnvPanel.Infrastructure/Persistence/Migrations/AppDbContextModelSnapshot.cs

@@ -519,6 +519,9 @@ namespace PnvPanel.Infrastructure.Persistence.Migrations
                         .ValueGeneratedOnAdd()
                         .HasColumnType("uuid");
 
+                    b.Property<int?>("PricePerConfigPerHalfYear")
+                        .HasColumnType("integer");
+
                     b.Property<int?>("PricePerConfigPerQuarter")
                         .HasColumnType("integer");