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/Abstractions/Persistence/IUsuarioRepository.cs

@@ -6,4 +6,6 @@ public interface IUsuarioRepository
 {
     Task<Usuario?> GetByUsernameAsync(string username);
     Task<Usuario?> GetByIdAsync(int id, CancellationToken ct = default);
+    Task<bool> ExistsByUsernameAsync(string username, CancellationToken ct = default);
+    Task<int> AddAsync(Usuario usuario, CancellationToken ct = default);
 }