feat(domain): ProductPrice entity + exceptions (PRD-003)

src/api/SIGCM2.Domain/Exceptions/ProductSinPrecioActivoException.cs

@@ -0,0 +1,18 @@
+namespace SIGCM2.Domain.Exceptions;
+
+/// <summary>
+/// Thrown when no ProductPrice row covers the requested date for the given product. → HTTP 404
+/// Consumers of IProductPricingService may throw this when GetPriceAtAsync returns null.
+/// </summary>
+public sealed class ProductSinPrecioActivoException : DomainException
+{
+    public int ProductId { get; }
+    public DateOnly Date { get; }
+
+    public ProductSinPrecioActivoException(int productId, DateOnly date)
+        : base($"No existe precio registrado para el producto {productId} aplicable a la fecha {date:yyyy-MM-dd}.")
+    {
+        ProductId = productId;
+        Date = date;
+    }
+}