feat(api): pagination on GET product prices (closes #47)

- GET /api/v1/products/{id}/prices now returns PagedResult<ProductPriceDto>
  with OFFSET/FETCH + COUNT via Dapper (two queries on same connection)
- Query params: ?page (default 1) and ?pageSize (default 20, max 100)
- Clamping: Math.Max(1, page) + Math.Clamp(pageSize, 1, 100) in handler
- Auth upgraded from [Authorize] to [RequirePermission("catalogo:productos:gestionar")]
- IProductPriceRepository.GetByProductIdAsync signature updated to paginated form
- AddProductPriceCommandHandler adapted to read back via page=1, pageSize=2
- TDD cycle: RED (tests updated to PagedResult shape) -> GREEN (implementation) -> REFACTOR
- Tests: 1418 total (1106 Application + 312 Api), 0 failures

closes #47

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

@@ -1,3 +1,4 @@
+using SIGCM2.Application.Common;
 using SIGCM2.Domain.Entities;
 
 namespace SIGCM2.Application.Abstractions.Persistence;
@@ -21,11 +22,14 @@ public interface IProductPriceRepository
         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.
+    /// Returns a paginated page of price rows for the product, ordered descending by PriceValidFrom.
+    /// Caller is responsible for clamping page (≥ 1) and pageSize (1–100) before calling.
+    /// Returns PagedResult with empty Items when the product has no price history or page is beyond total.
     /// </summary>
-    Task<IReadOnlyList<ProductPrice>> GetByProductIdAsync(
+    Task<PagedResult<ProductPrice>> GetByProductIdAsync(
         int productId,
+        int page,
+        int pageSize,
         CancellationToken ct = default);
 
     /// <summary>