feat(audit): enchufar audit en handlers de Usuario — Closes #6

7 command handlers del módulo Usuarios ahora auditan via IAuditLogger:

| Handler                                 | Action                  |
|-----------------------------------------|-------------------------|
| CreateUsuarioCommandHandler             | usuario.create          |
| UpdateUsuarioCommandHandler             | usuario.update          |
| DeactivateUsuarioCommandHandler         | usuario.deactivate      |
| ReactivateUsuarioCommandHandler         | usuario.reactivate      |
| ChangeMyPasswordCommandHandler          | usuario.password_change |
| ResetUsuarioPasswordCommandHandler      | usuario.password_reset  |
| UpdateUsuarioPermisosOverridesHandler   | usuario.permisos_update |

Patrón por handler (per design #D-1):
  using (var tx = new TransactionScope(Required, ReadCommitted, AsyncFlowEnabled))
  {
      await repo.UpdateAsync(...);
      await audit.LogAsync(...);
      tx.Complete();
  }
  // post-commit reads OUTSIDE the using block
  var updated = await repo.GetDetailAsync(...);

Metadata captured:
- usuario.create: after={username, nombre, apellido, email, rol} — NO password.
- usuario.update: {before, after} diff of editable fields.
- usuario.password_reset: {targetId} only — tempPassword is NEVER persisted to
  audit (returned to caller once, never stored).
- usuario.permisos_update: {before, after} of grant/deny override lists.

Key fix during implementation: initially used 'using var tx = ...' (bare
declaration). This kept the TransactionScope active for the rest of the method,
causing 'The current TransactionScope is already complete' when post-commit
reads (GetDetailAsync) tried to enlist. Solution: explicit 'using (var tx = ...)
{ ... }' block that disposes the scope before post-commit reads.

AuditContextMissingException surfaces from AuditLogger when IAuditContext
lacks ActorUserId — fail-closed per #REQ-AUD-4. In integration tests, the
middleware populates ActorUserId from the JWT sub of the authenticated admin.

Test updates: 6 existing unit test classes now inject IAuditLogger mock:
- CreateUsuarioCommandHandlerTests
- UpdateUsuarioCommandHandlerTests
- DeactivateUsuarioCommandHandlerTests
- ReactivateUsuarioCommandHandlerTests
- ChangeMyPasswordCommandHandlerTests
- ResetUsuarioPasswordCommandHandlerTests

Follow-up #6 ([Auditoría] Registrar admin creador en alta de usuarios) is
closed: CreateUsuarioCommandHandler now records ActorUserId = admin JWT sub
on every user creation. TODO comment removed.

Suite: 378/378 Application.Tests + 141/141 Api.Tests = 519/519 passing.

Closes #6
Refs: sdd/udt-010-auditoria-trazabilidad/{spec#REQ-UM-AUD, design, tasks#B7}

tests/SIGCM2.Application.Tests/Usuarios/Create/CreateUsuarioCommandHandlerTests.cs

@@ -1,6 +1,7 @@
 using NSubstitute;
 using SIGCM2.Application.Abstractions.Persistence;
 using SIGCM2.Application.Abstractions.Security;
+using SIGCM2.Application.Audit;
 using SIGCM2.Application.Usuarios.Create;
 using SIGCM2.Domain.Entities;
 using SIGCM2.Domain.Exceptions;
@@ -11,6 +12,7 @@ public class CreateUsuarioCommandHandlerTests
 {
     private readonly IUsuarioRepository _repository = Substitute.For<IUsuarioRepository>();
     private readonly IPasswordHasher _hasher = Substitute.For<IPasswordHasher>();
+    private readonly IAuditLogger _audit = Substitute.For<IAuditLogger>();
     private readonly CreateUsuarioCommandHandler _handler;
 
     private static CreateUsuarioCommand ValidCommand() => new(
@@ -23,7 +25,7 @@ public class CreateUsuarioCommandHandlerTests
 
     public CreateUsuarioCommandHandlerTests()
     {
-        _handler = new CreateUsuarioCommandHandler(_repository, _hasher);
+        _handler = new CreateUsuarioCommandHandler(_repository, _hasher, _audit);
     }
 
     // ── exists → throws ──────────────────────────────────────────────────────