feat(medios,secciones): application layer + handlers TDD — ADM-001 B3+B4

- IMedioRepository, ISeccionRepository interfaces
- MediosQuery, SeccionesQuery common records
- TipoSeccion static AllowedTipos helper
- Medios: 6 use cases (Create/Update/Deactivate/Reactivate/List/GetById) with validators, handlers and DTOs
- Secciones: 6 use cases mirroring Medios; Create validates MedioId active via IMedioRepository
- 52 unit tests (xUnit + NSubstitute) all green; audit LogAsync asserted per mutating handler
- DI registrations for all 12 handlers and validators auto-scanned via AddValidatorsFromAssemblyContaining

src/api/SIGCM2.Application/Secciones/Reactivate/ReactivateSeccionCommandHandler.cs

@@ -0,0 +1,49 @@
+using System.Transactions;
+using SIGCM2.Application.Abstractions;
+using SIGCM2.Application.Abstractions.Persistence;
+using SIGCM2.Application.Audit;
+using SIGCM2.Application.Secciones.Deactivate;
+using SIGCM2.Domain.Entities;
+using SIGCM2.Domain.Exceptions;
+
+namespace SIGCM2.Application.Secciones.Reactivate;
+
+public sealed class ReactivateSeccionCommandHandler : ICommandHandler<ReactivateSeccionCommand, SeccionStatusDto>
+{
+    private readonly ISeccionRepository _repo;
+    private readonly IAuditLogger _audit;
+
+    public ReactivateSeccionCommandHandler(ISeccionRepository repo, IAuditLogger audit)
+    {
+        _repo = repo;
+        _audit = audit;
+    }
+
+    public async Task<SeccionStatusDto> Handle(ReactivateSeccionCommand command)
+    {
+        var target = await _repo.GetByIdAsync(command.Id)
+            ?? throw new SeccionNotFoundException(command.Id);
+
+        // Idempotent: already active → return as-is without writing an audit event
+        if (target.Activo)
+            return new SeccionStatusDto(target.Id, target.Codigo, target.Activo);
+
+        var updated = target.WithActivo(true);
+
+        using var tx = new TransactionScope(
+            TransactionScopeOption.Required,
+            new TransactionOptions { IsolationLevel = System.Transactions.IsolationLevel.ReadCommitted },
+            TransactionScopeAsyncFlowOption.Enabled);
+
+        await _repo.UpdateAsync(updated);
+
+        await _audit.LogAsync(
+            action: "seccion.reactivate",
+            targetType: "Seccion",
+            targetId: command.Id.ToString());
+
+        tx.Complete();
+
+        return new SeccionStatusDto(updated.Id, updated.Codigo, updated.Activo);
+    }
+}