Add carrier field to Refund model and update Stripe views

Introduces a OneToOneField for carrier in the Refund model to support future integration and refactoring. Adds and updates TODO comments for email template usage, error handling in Stripe webhooks, and clarifies logic placement in Stripe models.

backend/account/tasks.py

@@ -10,6 +10,7 @@ from .models import CustomUser
 
 logger = get_task_logger(__name__)
 
+# TODO: předělat funkci ať funguje s base.html šablonou na emaily !!!
 def send_email_with_context(recipients, subject, message=None, template_name=None, html_template_name=None, context=None):
     """
     General function to send emails with a specific context.