Add MediatR commands/queries/handlers and DTOs

src/Printbase.Application/Products/Dtos/ProductVariantDto.cs

@@ -0,0 +1,17 @@
+namespace Printbase.Application.Products.Dtos;
+
+public class ProductVariantDto
+{
+    public Guid Id { get; set; }
+    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; }
+    public ProductDto? Product { get; set; }
+    public DateTime CreatedAt { get; set; }
+    public DateTime ModifiedAt { get; set; }
+}