Refactor sales pipeline: ShopPO, Invoice, SysConfig models

Major refactor replacing Sale/Order with ShopPO → Invoice → LoadingSheet flow. Removed legacy Sale/Order models and features; introduced ShopPO, ShopPOItem, Invoice, LoadingSheetOrder, and SysConfig models. Updated related entities, enums, repositories, and controllers. Added new migrations for schema changes and system config. Overhauled business logic and DI registrations. Updated documentation with migration plan.

Distribution/Features/Payment/PaymentDTOs.cs

@@ -1,23 +1,15 @@
-using Distribution.Models;
+using Distribution.Models;
 
 namespace Distribution.Features.Payment;
 
 public record CreatePaymentRequest(
-    decimal Amount,
-    PaymentMethod Method,
-    // TEMPORARY: auth middleware doesn't resolve a logged-in user yet — see testbug.md #28.
-    Guid? CollectedByUserId = null);
-
-public record CreateFlatPaymentRequest(
-    Guid SaleId,
     decimal Amount,
     PaymentMethod Method,
     Guid? CollectedByUserId = null);
 
 public record PaymentResponse(
     Guid Id,
-    Guid SaleId,
-    Guid StoreId,
+    Guid InvoiceId,
     decimal Amount,
     PaymentMethod Method,
     DateTime CollectedAt,