Refactored discount application logic in OrderItem to support multiple coupons with configurable multiplication and addition rules from ShopConfiguration. Updated DiscountCode model to use PositiveIntegerField for percent and improved validation. Extended ShopConfiguration with new fields for contact, social media, and coupon settings. Ensured ShopConfiguration singleton creation at app startup.
Major refactor of commerce models: restructured Carrier, Payment, and DiscountCode models, improved order total calculation, and integrated Zasilkovna and Stripe logic. Added new configuration Django app for shop settings, updated Zasilkovna and Stripe models, and fixed Zasilkovna client WSDL URL. Removed unused serializers and views in commerce, and registered new apps in settings.
