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/Abstractions/Persistence/IMedioRepository.cs

@@ -0,0 +1,13 @@
+using SIGCM2.Application.Common;
+using SIGCM2.Domain.Entities;
+
+namespace SIGCM2.Application.Abstractions.Persistence;
+
+public interface IMedioRepository
+{
+    Task<int> AddAsync(Medio m, CancellationToken ct = default);
+    Task<Medio?> GetByIdAsync(int id, CancellationToken ct = default);
+    Task<bool> ExistsByCodigoAsync(string codigo, CancellationToken ct = default);
+    Task UpdateAsync(Medio m, CancellationToken ct = default);
+    Task<PagedResult<Medio>> GetPagedAsync(MediosQuery q, CancellationToken ct = default);
+}

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

@@ -0,0 +1,13 @@
+using SIGCM2.Application.Common;
+using SIGCM2.Domain.Entities;
+
+namespace SIGCM2.Application.Abstractions.Persistence;
+
+public interface ISeccionRepository
+{
+    Task<int> AddAsync(Seccion s, CancellationToken ct = default);
+    Task<Seccion?> GetByIdAsync(int id, CancellationToken ct = default);
+    Task<bool> ExistsByCodigoInMedioAsync(int medioId, string codigo, CancellationToken ct = default);
+    Task UpdateAsync(Seccion s, CancellationToken ct = default);
+    Task<PagedResult<Seccion>> GetPagedAsync(SeccionesQuery q, CancellationToken ct = default);
+}