fix: add account_tiers and admin_settings tables to payment migrations

- Fixed lost account tiers after server restart
- Fixed lost payment gateway configurations on MySQL
- Added _create_account_tiers_table() method to create tiers table
- Added admin_settings table to _create_config_tables()
- Added default free tier insertion if none exists
- Both fixes use CREATE TABLE IF NOT EXISTS to preserve existing data
- Resolves issues introduced in commits f997a0fb and 0052431f