Implements automatic subscription renewal processing with:
- RenewalProcessor class that finds and processes due subscriptions
- Extends billing periods by 30 days (monthly) or 365 days (yearly)
- Applies pending tier changes (downgrades) at renewal
- Handles subscription cancellations (cancel_at_period_end flag)
- Supports both fiat (card/PayPal) and crypto wallet payments
- Graceful handling of payment failures
- Comprehensive test coverage (8 tests, all passing)

Files:
- aisbf/payments/subscription/renewal.py: Core renewal processor
- tests/payments/test_renewal.py: Complete test suite
- aisbf/payments/subscription/__init__.py: Export RenewalProcessor