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/business/billing/config.py

@@ -0,0 +1,37 @@
+"""
+Billing configuration settings
+"""
+from django.conf import settings
+
+# Payment Gateway Mode
+PAYMENT_GATEWAY_MODE = getattr(settings, 'PAYMENT_GATEWAY_MODE', 'sandbox')  # 'sandbox' or 'production'
+
+# Auto-approve payments (development only)
+AUTO_APPROVE_PAYMENTS = getattr(settings, 'AUTO_APPROVE_PAYMENTS', False)
+
+# Invoice due date offset (days)
+INVOICE_DUE_DATE_OFFSET = getattr(settings, 'INVOICE_DUE_DATE_OFFSET', 7)
+
+# Grace period for payment (days)
+PAYMENT_GRACE_PERIOD = getattr(settings, 'PAYMENT_GRACE_PERIOD', 7)
+
+# Maximum payment retry attempts
+MAX_PAYMENT_RETRIES = getattr(settings, 'MAX_PAYMENT_RETRIES', 3)
+
+# Subscription renewal advance notice (days)
+SUBSCRIPTION_RENEWAL_NOTICE_DAYS = getattr(settings, 'SUBSCRIPTION_RENEWAL_NOTICE_DAYS', 7)
+
+# Default subscription plan slugs
+DEFAULT_PLAN_SLUGS = {
+    'free': getattr(settings, 'FREE_PLAN_SLUG', 'basic-free'),
+    'starter': getattr(settings, 'STARTER_PLAN_SLUG', 'starter-10'),
+    'professional': getattr(settings, 'PROFESSIONAL_PLAN_SLUG', 'professional-100'),
+    'enterprise': getattr(settings, 'ENTERPRISE_PLAN_SLUG', 'enterprise-unlimited'),
+}
+
+# Credit package slugs
+DEFAULT_CREDIT_PACKAGES = {
+    'small': getattr(settings, 'SMALL_CREDIT_PACKAGE_SLUG', 'credits-100'),
+    'medium': getattr(settings, 'MEDIUM_CREDIT_PACKAGE_SLUG', 'credits-500'),
+    'large': getattr(settings, 'LARGE_CREDIT_PACKAGE_SLUG', 'credits-1000'),
+}