chore(infra): configure coverlet for backend C# coverage

Add coverlet.runsettings with Cobertura format, exclusions for migrations,
DI wiring, Program.cs and auto-props. Document coverage commands in README.

coverlet.collector 6.0.4 was already present via Directory.Packages.props.

Coverage baseline (Application.Tests + Api.Tests combined):
- Application.Tests: line 80.9%, branch 65.3%
- Api.Tests: line 64.9%, branch 57.8%

Closes #48

README.md

@@ -73,6 +73,27 @@ dotnet test tests/SIGCM2.Api.Tests               # integration (requiere SIGCM2_
 cd src/web && npx vitest run
 ```
 
+### Coverage (backend)
+
+```bash
+# Generar reporte de coverage en formato Cobertura
+dotnet test --collect:"XPlat Code Coverage" --settings coverlet.runsettings --results-directory ./TestResults
+```
+
+El comando genera un `coverage.cobertura.xml` por cada proyecto de test en `./TestResults/`.
+
+Para convertirlo a HTML:
+
+```bash
+# Instalar ReportGenerator (solo la primera vez)
+dotnet tool install -g dotnet-reportgenerator-globaltool
+
+# Generar reporte HTML
+reportgenerator -reports:"./TestResults/**/coverage.cobertura.xml" -targetdir:"./coverage-report" -reporttypes:Html
+```
+
+Abrí `./coverage-report/index.html` en el browser para ver el detalle por archivo.
+
 ## Convenciones
 
 - Ramas: `feature/UDT-XXX` desde `main`.