feat(user-center): add cash wallet balance separate from credits

Introduce AKSRT_Balance (user_meta aksrt_balance + balance_log) for
Zibll-like pay_balance wallet. User center assets panel shows wallet
and points; balance gateway/card-pass/refunds credit cash when enabled.

inc/classes/db-traits/trait-aksrt-db-migrator-part-3.php

@@ -11,6 +11,7 @@ trait AKSRT_DB_Migrator_Part_3 {
         global $wpdb;
         $allowed = [
             'credit_log'     => 'aksrt_credit_log',
+            'balance_log'    => 'aksrt_balance_log',
             'signin_log'     => 'aksrt_signin_log',
             'invite'         => 'aksrt_invite',
             'dl_token'       => 'aksrt_dl_token',