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/ServicesExternos.API.csproj

@@ -0,0 +1,19 @@
+<Project Sdk="Microsoft.NET.Sdk.Web">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Dapper" Version="2.1.79" />
+    <PackageReference Include="MailKit" Version="4.17.0" />
+    <PackageReference Include="Microsoft.AspNetCore.OpenApi" Version="10.0.2" />
+    <PackageReference Include="Newtonsoft.Json" Version="13.0.4" />
+    <PackageReference Include="Npgsql" Version="10.0.3" />
+  </ItemGroup>
+
+
+
+</Project>