feat(billing): add missing payment methods and configurations

- Added migration to include global payment method configurations for Stripe and PayPal (both disabled).
- Ensured existing payment methods like bank transfer and manual payment are correctly configured.
- Added database constraints and indexes for improved data integrity in billing models.
- Introduced foreign key relationship between CreditTransaction and Payment models.
- Added webhook configuration fields to PaymentMethodConfig for future payment gateway integrations.
- Updated SignUpFormUnified component to handle payment method selection based on user country and plan.
- Implemented PaymentHistory component to display user's payment history with status indicators.

backend/igny8_core/auth/models.py

@@ -249,8 +249,6 @@ class Subscription(models.Model):
         'igny8_core_auth.Plan',
         on_delete=models.PROTECT,
         related_name='subscriptions',
-        null=True,
-        blank=True,
         help_text='Subscription plan (tracks historical plan even if account changes plan)'
     )
     stripe_subscription_id = models.CharField(