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/ShopPO/ShopPODTOs.cs

@@ -0,0 +1,24 @@
+using Distribution.Models;
+
+namespace Distribution.Features.ShopPO;
+
+public record CreateShopPORequest(Guid StoreId, List<ShopPOItemRequest> Items);
+public record ShopPOItemRequest(Guid ItemId, int Quantity, decimal? UnitPrice = null);
+public record UpdateShopPOItemsRequest(List<ShopPOItemRequest> Items);
+
+public record ShopPOItemResponse(Guid Id, Guid ItemId, string ItemName, int Quantity, int QuantityFulfilled, decimal UnitPrice, bool IsActive);
+
+public record ShopPOResponse(
+    Guid Id,
+    Guid StoreId,
+    string StoreName,
+    Guid RaisedById,
+    Guid? InvoiceId,
+    decimal TotalAmount,
+    ShopPOStatus Status,
+    DateTime? ConfirmedAt,
+    Guid? ConfirmedBy,
+    DateTime? FulfilledAt,
+    List<ShopPOItemResponse> Items,
+    bool IsActive,
+    DateTime CreatedAt);