- Introduced SaleAbstractModel and SaleLineAbstractModel as abstract bases
- Added CatalogSale and CatalogSaleLine models inheriting from them
- Created migration 0045 for new models
- Added CatalogSaleView, CatalogSaleSerializer with nested line creation
- Registered new models in admin
- Added catalog_sales router endpoint to URLs
- Removed placeholder api/ package (now redundant)