- Add DatabaseManager.placeholder property returning '?' (SQLite) or '%s' (MySQL)
- Fix paypal_handler.py: payment_methods INSERT
- Fix service.py: payment_methods INSERT for PayPal vault
- Fix subscription/manager.py: all SQL in create/upgrade/downgrade/cancel_subscription
- Fix subscription/renewal.py: all SQL in process_renewals/_renew_subscription/_cancel_subscription
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>