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

coverlet.runsettings

@@ -0,0 +1,48 @@
+<?xml version="1.0" encoding="utf-8"?>
+<RunSettings>
+  <!--
+    Configuracion de coverage con coverlet.collector.
+    Uso: dotnet test /collect:"XPlat Code Coverage" /settings:coverlet.runsettings /results-directory:./TestResults
+  -->
+  <RunConfiguration>
+    <!-- Mantener ejecución secuencial (hereda política de tests.runsettings) -->
+    <MaxCpuCount>1</MaxCpuCount>
+  </RunConfiguration>
+
+  <DataCollectionRunSettings>
+    <DataCollectors>
+      <DataCollector friendlyName="XPlat Code Coverage">
+        <Configuration>
+          <!-- Formato de salida: cobertura (compatible con ReportGenerator y CI/CD) -->
+          <Format>cobertura</Format>
+
+          <!-- Exclusiones por atributo generado -->
+          <ExcludeByAttribute>GeneratedCodeAttribute,ExcludeFromCodeCoverageAttribute</ExcludeByAttribute>
+
+          <!-- Exclusiones por tipo/namespace -->
+          <Exclude>
+            <!-- Migrations embebidas (SQL scripts, no lógica de negocio) -->
+            [*.Migrations]*,
+            <!-- Los proyectos de test no se miden a sí mismos -->
+            [*.Tests]*,
+            [SIGCM2.TestSupport]*,
+            <!-- Program.cs: host wiring, no testeable unitariamente -->
+            [SIGCM2.Api]Program,
+            <!-- Extension methods de DI: una línea por registro, ruido sin valor -->
+            [*]*.Extensions.*Extensions,
+            [*]*.DependencyInjection
+          </Exclude>
+
+          <!-- No medir las propiedades auto-implementadas -->
+          <SkipAutoProps>true</SkipAutoProps>
+
+          <!-- No incluir el assembly de tests en el reporte -->
+          <IncludeTestAssembly>false</IncludeTestAssembly>
+
+          <!-- Permitir timestamps reales en el reporte (no forzar determinismo) -->
+          <DeterministicReport>false</DeterministicReport>
+        </Configuration>
+      </DataCollector>
+    </DataCollectors>
+  </DataCollectionRunSettings>
+</RunSettings>