feat: core entities, infrastructure, and lead repositories

- 25 domain entities mapped from original Datos/ classes
- 4 enums (EstadoLead, TipoPago, TipoDocumento, TipoDescuento)
- DTOs for Lead, Auth flows
- 6 repository interfaces
- VentasDbContext with EF Core + Npgsql
- LeadRepository (command SPs) and LeadQueryRepository (query SPs) with Dapper
- Program.cs with JWT auth, CORS, DI setup
- appsettings.json with connection string
- .gitignore for bin/obj/node_modules
- ROADMAP.md for daily progress tracking

backend/Ventas.slnx

@@ -0,0 +1,8 @@
+<Solution>
+  <Folder Name="/src/">
+    <Project Path="src/Ventas.API/Ventas.API.csproj" />
+    <Project Path="src/Ventas.Core/Ventas.Core.csproj" />
+    <Project Path="src/Ventas.Infrastructure/Ventas.Infrastructure.csproj" />
+    <Project Path="src/Ventas.Services/Ventas.Services.csproj" />
+  </Folder>
+</Solution>