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/Auth/AuthOptions.cs

@@ -9,4 +9,9 @@ public sealed class AuthOptions
 {
     public int AccessTokenMinutes { get; set; } = 60;
     public int RefreshTokenDays { get; set; } = 7;
+
+    // Password policy — configurable, secure defaults
+    public int PasswordMinLength { get; set; } = 8;
+    public bool PasswordRequireLetter { get; set; } = true;
+    public bool PasswordRequireDigit { get; set; } = true;
 }