Refactor commerce models and enhance payment logic

Refactored commerce models to remove language prefixes from status choices, improved order and payment validation, and enforced business rules for payment and shipping combinations. Updated order item and cart calculations to use VAT-inclusive prices, added unique constraints and indexes to reviews, and improved stock management logic. Added new Stripe client methods for session and refund management, and updated Zasilkovna and Deutsche Post models for consistency. Minor fixes and improvements across related tasks, URLs, and configuration models.

backend/configuration/models.py

@@ -44,8 +44,8 @@ class SiteConfiguration(models.Model):
     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", "cz#Czech Koruna"
-        EUR = "EUR", "cz#Euro"
+        CZK = "CZK", "Czech Koruna"
+        EUR = "EUR", "Euro"
     currency = models.CharField(max_length=10, default=CURRENCY.CZK, choices=CURRENCY.choices)
 
     class Meta:
@@ -87,6 +87,10 @@ class VATRate(models.Model):
         default=False,
         help_text="Default rate for new products"
     )
+    is_active = models.BooleanField(
+        default=True,
+        help_text="Whether this VAT rate is active and available for use"
+    )
     created_at = models.DateTimeField(auto_now_add=True)
     
     class Meta: