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.Domain/Entities/TipoSeccion.cs

@@ -0,0 +1,10 @@
+namespace SIGCM2.Domain.Entities;
+
+/// <summary>
+/// Allowed string values for Seccion.Tipo.
+/// Enforced at application layer (validator) and database layer (CHECK constraint).
+/// </summary>
+public static class TipoSeccion
+{
+    public static readonly string[] AllowedTipos = { "clasificados", "notables", "suplementos" };
+}