feat(api): UDT-003 registro de usuarios — backend completo (Phases 1-6)

- Domain: Usuario.ForCreation factory, UsernameAlreadyExistsException, IUsuarioRepository extendido
- Application: CreateUsuarioCommand/Validator/Handler, UsuarioCreatedDto, AuthOptions password policy
- Infrastructure: UsuarioRepository.ExistsByUsernameAsync + AddAsync (INSERT OUTPUT INSERTED.Id), RoleClaimType="rol" en TokenValidationParameters
- Api: UsuariosController POST api/v1/users [Authorize(Roles="admin")], ExceptionFilter mapea UsernameAlreadyExistsException + SqlException 2627 → 409
- Tests (unit): 43 tests — 33 validator + 10 handler (107 total, green)
- Tests (integration): 7 tests CreateUsuarioEndpoint — 401/403/400/201/409/race/e2e (green)
- Fix: TestWebAppFactory.ConfigureTestServices reemplaza SqlConnectionFactory singleton con CS de test correcto

src/api/SIGCM2.Application/DependencyInjection.cs

@@ -4,6 +4,7 @@ using SIGCM2.Application.Abstractions;
 using SIGCM2.Application.Auth.Login;
 using SIGCM2.Application.Auth.Logout;
 using SIGCM2.Application.Auth.Refresh;
+using SIGCM2.Application.Usuarios.Create;
 
 namespace SIGCM2.Application;
 
@@ -15,6 +16,7 @@ public static class DependencyInjection
         services.AddScoped<ICommandHandler<LoginCommand, LoginResponseDto>, LoginCommandHandler>();
         services.AddScoped<ICommandHandler<RefreshCommand, RefreshResponseDto>, RefreshCommandHandler>();
         services.AddScoped<ICommandHandler<LogoutCommand, LogoutResponseDto>, LogoutCommandHandler>();
+        services.AddScoped<ICommandHandler<CreateUsuarioCommand, UsuarioCreatedDto>, CreateUsuarioCommandHandler>();
 
         // FluentValidation validators (scans entire Application assembly)
         services.AddValidatorsFromAssemblyContaining<LoginCommandValidator>();