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

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

@@ -0,0 +1,23 @@
+namespace SIGCM2.Domain.Exceptions;
+
+/// <summary>
+/// Thrown when attempting to add a ProductPrice with a PriceValidFrom that is not strictly
+/// greater than the currently active price's PriceValidFrom. → HTTP 409
+/// </summary>
+public sealed class ProductPriceForwardOnlyException : DomainException
+{
+    public int ProductId { get; }
+    public DateOnly NewPriceValidFrom { get; }
+    public DateOnly ActivePriceValidFrom { get; }
+
+    public ProductPriceForwardOnlyException(
+        int productId,
+        DateOnly newPriceValidFrom,
+        DateOnly activePriceValidFrom)
+        : base($"El nuevo PriceValidFrom ({newPriceValidFrom:yyyy-MM-dd}) debe ser estrictamente mayor al PriceValidFrom del precio activo ({activePriceValidFrom:yyyy-MM-dd}).")
+    {
+        ProductId = productId;
+        NewPriceValidFrom = newPriceValidFrom;
+        ActivePriceValidFrom = activePriceValidFrom;
+    }
+}