refactor: move payment gateway settings to payment settings page and hide API Tokens for config admin

- Moved payment gateway configuration (Stripe, PayPal, crypto) from admin tiers page to admin payment settings page
- Removed payment gateway HTML section (lines 115-290) from admin_tiers.html
- Removed payment gateway JavaScript functions from admin_tiers.html
- Added payment gateway section to admin_payment_settings.html with all 6 gateways (PayPal, Stripe, BTC, ETH, USDT, USDC)
- Added loadPaymentGateways() and savePaymentGateways() functions to admin_payment_settings.html
- Hide 'API Tokens' navigation link for config admin user (user_id is null for config admin)
- API Tokens link now only shows for database users (request.session.user_id exists)
- Config admin defined in aisbf.json has no user_id, so they won't see API Tokens link
- Admin tiers page now focuses only on tier management and currency settings
- Admin payment settings page now has all payment configuration in one place