feat: fase 2 - services externos (dte, transbank, email)

- New solution ServicesExternos.slnx with standalone API project
- DteService + DteController (LibreDTE: emitir, generar PDF, consultar folio)
- TransbankService + TransbankController (crear transacción, confirmar, voucher SP)
- EmailService + EmailController (MailKit: send with HTML + attachments)
- Models for DTE, Transbank, Email requests/responses
- Dockerfile + appsettings.json with all service configs
- HttpClient factory for Transbank and LibreDTE
- Build: 0 errors

services-externos/src/ServicesExternos.API/Models/TransbankModels.cs

@@ -0,0 +1,26 @@
+namespace ServicesExternos.API.Models;
+
+public class TransbankTransactionRequest
+{
+    public string? BuyOrder { get; set; }
+    public string? SessionId { get; set; }
+    public int Amount { get; set; }
+    public string? ReturnUrl { get; set; }
+}
+
+public class TransbankConfirmRequest
+{
+    public string? TokenWs { get; set; }
+}
+
+public class VoucherRequest
+{
+    public string Token { get; set; } = string.Empty;
+    public string AccountingDate { get; set; } = string.Empty;
+    public string BuyOrder { get; set; } = string.Empty;
+    public string CardNumber { get; set; } = string.Empty;
+    public string AuthorizationCode { get; set; } = string.Empty;
+    public string PaymentType { get; set; } = string.Empty;
+    public string SharesNumber { get; set; } = string.Empty;
+    public int Amount { get; set; }
+}