Add MediatR commands/queries/handlers and DTOs

src/Printbase.Application/Products/Commands/CreateProductVariantCommand.cs

@@ -0,0 +1,16 @@
+using MediatR;
+using Printbase.Application.Products.Dtos;
+
+namespace Printbase.Application.Products.Commands;
+
+public class CreateProductVariantCommand : IRequest<ProductVariantDto>
+{
+    public Guid ProductId { get; set; }
+    public string Size { get; set; } = null!;
+    public string? Color { get; set; }
+    public decimal Price { get; set; }
+    public string? ImageUrl { get; set; }
+    public string Sku { get; set; } = null!;
+    public int StockQuantity { get; set; }
+    public bool IsActive { get; set; } = true;
+}