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.API/Properties/launchSettings.json

@@ -0,0 +1,14 @@
+{
+  "$schema": "https://json.schemastore.org/launchsettings.json",
+  "profiles": {
+    "http": {
+      "commandName": "Project",
+      "dotnetRunMessages": true,
+      "launchBrowser": false,
+      "applicationUrl": "http://localhost:5087",
+      "environmentVariables": {
+        "ASPNETCORE_ENVIRONMENT": "Development"
+      }
+    }
+  }
+}