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/src/Ventas.Core/Interfaces/IUsuarioRepository.cs

@@ -0,0 +1,8 @@
+namespace Ventas.Core.Interfaces;
+
+public interface IUsuarioRepository
+{
+    Task<string> BuscarAsync(string usuario, string clave);
+    Task<string> InfoAsync(string usuario);
+    Task<string> PerfilAsync(string usuario);
+}