Add sales invoice document

Backend/ERPCore/Program.cs

@@ -89,6 +89,9 @@ builder.Services.AddScoped<IFifoCostingService, FifoCostingService>();
 builder.Services.AddScoped<IStockService, StockService>();
 builder.Services.AddScoped<IGrnService, GrnService>();
 
+// Sales (Phase 1)
+builder.Services.AddScoped<ISalesInvoiceService, SalesInvoiceService>();
+
 // Stock transactions + reference data (docs/11 §5–6)
 builder.Services.AddScoped<IStockMutator, StockMutator>();
 builder.Services.AddScoped<IReasonCodeService, ReasonCodeService>();