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/models.py

@@ -5,10 +5,30 @@ from django.db import models
 class ShopConfiguration(models.Model):
     name = models.CharField(max_length=100, default="Shop name", unique=True)
 
+    logo = models.ImageField(upload_to='shop_logos/', blank=True, null=True)
+    favicon = models.ImageField(upload_to='shop_favicons/', blank=True, null=True)
+
+    contact_email = models.EmailField(max_length=254, blank=True, null=True)
+    contact_phone = models.CharField(max_length=20, blank=True, null=True)
+    contact_address = models.TextField(blank=True, null=True)
+    opening_hours = models.JSONField(blank=True, null=True) #FIXME: vytvoř tvar pro otvírací dobu
+
+    #Social
+    facebook_url = models.URLField(max_length=200, blank=True, null=True)
+    instagram_url = models.URLField(max_length=200, blank=True, null=True)
+    youtube_url = models.URLField(max_length=200, blank=True, null=True)
+    tiktok_url = models.URLField(max_length=200, blank=True, null=True)
+    whatsapp_number = models.CharField(max_length=20, blank=True, null=True)
+
+    #zasilkovna settings
     zasilkovna_shipping_price = models.DecimalField(max_digits=10, decimal_places=2, default=50)
     zasilkovna_address_id = models.CharField(max_length=100, blank=True, null=True, help_text="ID výdejního místa Zásilkovny pro odesílání zásilek")
     free_shipping_over = models.DecimalField(max_digits=10, decimal_places=2, default=2000)
-    
+
+    #coupon settings
+    multiplying_coupons = models.BooleanField(default=True, help_text="Násobení kupónů v objednávce (ano/ne), pokud ne tak se použije pouze nejvyšší slevový kupón")
+    addition_of_coupons_amount = models.BooleanField(default=False, help_text="Sčítání slevových kupónů v objednávce (ano/ne), pokud ne tak se použije pouze nejvyšší slevový kupón")
+
     class CURRENCY(models.TextChoices):
         CZK = "CZK", "Czech Koruna"
         EUR = "EUR", "Euro"