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

src/api/SIGCM2.Domain/Entities/ProductPrice.cs

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+namespace SIGCM2.Domain.Entities;
+
+/// <summary>
+/// PRD-003 — Immutable record representing a price snapshot for a Product.
+/// Business dates are Cat2 (civil Argentina dates, DateOnly). Forward-only — no mutations.
+/// PriceValidTo = null means the row is the currently active price.
+/// </summary>
+public sealed record ProductPrice(
+    long Id,
+    int ProductId,
+    decimal Price,
+    DateOnly PriceValidFrom,
+    DateOnly? PriceValidTo,
+    DateTime FechaCreacion)
+{
+    /// <summary>True if this row is the currently active price (PriceValidTo is null).</summary>
+    public bool IsActive => PriceValidTo is null;
+
+    /// <summary>
+    /// True if this price's window covers the given civil date (inclusive on both ends).
+    /// An active row (PriceValidTo = null) covers any date on or after PriceValidFrom.
+    /// </summary>
+    public bool CoversDate(DateOnly date)
+        => PriceValidFrom <= date && (PriceValidTo is null || PriceValidTo >= date);
+}

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

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+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;
+    }
+}

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

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+namespace SIGCM2.Domain.Exceptions;
+
+/// <summary>
+/// Thrown when a ProductPrice value fails domain business validation
+/// (e.g., Price &lt;= 0, PriceValidFrom in the past). → HTTP 400
+/// Used as defense-in-depth alongside FluentValidation in the Application layer.
+/// </summary>
+public sealed class ProductPriceInvalidException : DomainException
+{
+    public string Field { get; }
+    public string Reason { get; }
+
+    public ProductPriceInvalidException(string field, string reason)
+        : base($"Valor inválido para {field}: {reason}")
+    {
+        Field = field;
+        Reason = reason;
+    }
+}

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

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+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;
+    }
+}