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/class-aksrt-db-migrator.php

@@ -19,7 +19,7 @@ class AKSRT_DB_Migrator {
     use AKSRT_DB_Migrator_Part_3;
 
     /** Current schema version; changing it re-runs dbDelta. */
-    const SCHEMA_VERSION = '1.14.0';
+    const SCHEMA_VERSION = '1.15.0';
 
     const OPT_KEY = 'aksrt_db_schema_version';