Fix all test compilation errors caused by T400.10/T400.20/T400.30:
- Handler constructors: add TimeProvider.System as last argument
- Domain mutator calls: add DateTime.UtcNow as explicit 'now' argument
- AuditLogger/SecurityEventLogger Build() helpers: add TimeProvider.System
- JwtService test constructors: add TimeProvider.System
Cat2 coverage already present in TimeProviderArgentinaExtensionsTests.cs:
FakeTimeProvider proves GetArgentinaToday() returns ART civil date, not UTC.
- 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
