Fix: Remove unused Product import from Customer model

The import was left over from the removed loyalty system and could
cause issues during tenant creation when products tables don't exist yet.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

myproject/customers/models.py

@@ -2,7 +2,6 @@ import phonenumbers
 from django.core.exceptions import ValidationError
 from django.db import models
 from phonenumber_field.modelfields import PhoneNumberField
-from products.models import Product
 
 
 class Customer(models.Model):