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/Entities/Empresa.cs

@@ -0,0 +1,16 @@
+namespace Ventas.Core.Entities;
+
+public class Empresa
+{
+    public string Rut { get; set; } = string.Empty;
+    public string RazonSocial { get; set; } = string.Empty;
+    public string? Direccion { get; set; }
+    public string? ComunaId { get; set; }
+    public int? TipoId { get; set; }
+    public int? TamagnoId { get; set; }
+    public string? GiroNombre { get; set; }
+    public string? Contacto { get; set; }
+    public int? Fono { get; set; }
+    public string? Mail { get; set; }
+    public string? Origen { get; set; }
+}