feat(application): commands/queries + IProductPricingService (PRD-003)

- IProductPriceRepository (AddAsync/GetByProductIdAsync/GetActiveAsync)
- ProductPriceDto, AddProductPriceCommand/Response, GetProductPricesQuery
- AddProductPriceCommandValidator (FluentValidation + TimeProvider, fecha >= hoy_AR)
- AddProductPriceCommandHandler (TransactionScope AsyncFlow, audit fail-closed)
- GetProductPricesQueryHandler (verifica producto existe, lista vacía válida)
- IProductPricingService + ProductPricingService (GetPriceAtAsync → decimal?)
- DI wiring en DependencyInjection.cs
- 29 tests NSubstitute + FakeTimeProvider, 1081 Application.Tests GREEN

src/api/SIGCM2.Application/Abstractions/Persistence/IProductPriceRepository.cs

@@ -0,0 +1,40 @@
+using SIGCM2.Domain.Entities;
+
+namespace SIGCM2.Application.Abstractions.Persistence;
+
+/// <summary>
+/// PRD-003 — Write + query access to dbo.ProductPrices.
+/// Implemented by ProductPriceRepository (Dapper) in Infrastructure.
+/// </summary>
+public interface IProductPriceRepository
+{
+    /// <summary>
+    /// Invokes dbo.usp_AddProductPrice inside the ambient TransactionScope.
+    /// Returns (newId, closedId?). Throws:
+    ///   - ProductPriceForwardOnlyException on SQL THROW 50409 or unique index violation (2601/2627).
+    ///   - ProductNotFoundException on SQL THROW 50404.
+    /// </summary>
+    Task<(long NewId, long? ClosedId)> AddAsync(
+        int productId,
+        decimal price,
+        DateOnly priceValidFrom,
+        CancellationToken ct = default);
+
+    /// <summary>
+    /// Returns all price rows for the product, ordered descending by PriceValidFrom (active first).
+    /// Returns empty list when the product has no price history.
+    /// </summary>
+    Task<IReadOnlyList<ProductPrice>> GetByProductIdAsync(
+        int productId,
+        CancellationToken ct = default);
+
+    /// <summary>
+    /// Returns the ProductPrice row whose window [PriceValidFrom, PriceValidTo] covers the given
+    /// civil date, or null if no row matches (no history, or date is before any recorded price).
+    /// Used by ProductPricingService.GetPriceAtAsync.
+    /// </summary>
+    Task<ProductPrice?> GetActiveAsync(
+        int productId,
+        DateOnly date,
+        CancellationToken ct = default);
+}