Refactor commerce models and add configuration app

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.

backend/configuration/admin.py

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+from django.contrib import admin
+
+# Register your models here.

backend/configuration/apps.py

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+from django.apps import AppConfig
+
+
+class ConfigurationConfig(AppConfig):
+    default_auto_field = 'django.db.models.BigAutoField'
+    name = 'configuration'

backend/configuration/models.py

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+from django.db import models
+
+# Create your models here.
+
+class ShopConfiguration(models.Model):
+    name = models.CharField(max_length=100, default="Shop name", unique=True)
+
+    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)
+    
+    class CURRENCY(models.TextChoices):
+        CZK = "CZK", "Czech Koruna"
+        EUR = "EUR", "Euro"
+    currency = models.CharField(max_length=10, default=CURRENCY.CZK, choices=CURRENCY.choices)
+
+    class Meta:
+        verbose_name = "Shop Configuration"
+        verbose_name_plural = "Shop Configuration"
+
+    def save(self, *args, **kwargs):
+        # zajištění singletonu
+        self.pk = 1
+        super().save(*args, **kwargs)
+
+    @classmethod
+    def get_solo(cls):
+        obj, _ = cls.objects.get_or_create(pk=1)
+        return obj

backend/configuration/tests.py

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+from django.test import TestCase
+
+# Create your tests here.

backend/configuration/views.py

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+from django.shortcuts import render
+
+# Create your views here.