feat(BE-58,59,60,61): implement stock transfer API, validation, audit log, and schema update

- Build stock transfer API with nested details, deliveries, and items
- Extend DB schema for stock transfers
- Implement validation for transfer request and stock
- Prepare/implement transfer audit log structure
- Preload all relations for complete response
- Update DTOs for nested response
- Remove redundant root fields, use relation objects

internal/entities/stock-transfer.go

@@ -19,4 +19,5 @@ type StockTransfer struct {
 	ToWarehouse   *Warehouse              `gorm:"foreignKey:ToWarehouseId"`
 	Details       []StockTransferDetail   `gorm:"foreignKey:StockTransferId"`
 	Deliveries    []StockTransferDelivery `gorm:"foreignKey:StockTransferId"`
+	CreatedUser   *User                   `gorm:"foreignKey:CreatedBy"`
 }