Add choices API endpoint and OpenAPI client setup

Introduces a new /api/choices/ endpoint for fetching model choices with multilingual labels. Updates Django models to use 'cz#' prefix for Czech labels. Adds OpenAPI client generation via orval, refactors frontend API structure, and provides documentation and helper scripts for dynamic choices and OpenAPI usage.

backend/thirdparty/stripe/models.py

@@ -7,12 +7,12 @@ from .client import StripeClient
     
 class StripeModel(models.Model):
     class STATUS_CHOICES(models.TextChoices):
-        PENDING = "pending", "Čeká se na platbu"
-        PAID = "paid", "Zaplaceno"
-        FAILED = "failed", "Neúspěšné"
-        CANCELLED = "cancelled", "Zrušeno"
-        REFUNDING = "refunding", "Platba se vrací"
-        REFUNDED = "refunded", "Platba úspěšně vrácena"
+        PENDING = "pending", "cz#Čeká se na platbu"
+        PAID = "paid", "cz#Zaplaceno"
+        FAILED = "failed", "cz#Neúspěšné"
+        CANCELLED = "cancelled", "cz#Zrušeno"
+        REFUNDING = "refunding", "cz#Platba se vrací"
+        REFUNDED = "refunded", "cz#Platba úspěšně vrácena"
     
     status = models.CharField(max_length=20, choices=STATUS_CHOICES.choices, default=STATUS_CHOICES.PENDING)