Enhance discount logic and shop configuration models

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.

backend/configuration/apps.py

@@ -1,6 +1,21 @@
 from django.apps import AppConfig
-
+from django.db.utils import OperationalError, ProgrammingError
+from .models import ShopConfiguration
 
 class ConfigurationConfig(AppConfig):
     default_auto_field = 'django.db.models.BigAutoField'
     name = 'configuration'
+
+    def ready(self):
+        """Ensure the ShopConfiguration singleton exists at startup.
+
+        Wrapped in broad DB error handling so that commands like
+        makemigrations/migrate don't fail when the table does not yet exist.
+        """
+        try:
+            ShopConfiguration.get_solo()
+            
+        except (OperationalError, ProgrammingError):
+            ShopConfiguration.objects.create()
+
+