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-2.php

@@ -33,6 +33,25 @@ trait AKSRT_DB_Migrator_Part_2 {
         ) {$charset_collate};";
         dbDelta($sql_credit_log);
 
+        // ---------- 1b. 现金余额流水（站内钱包，与积分分离） ----------
+        $sql_balance_log = "CREATE TABLE {$prefix}aksrt_balance_log (
+            id BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
+            user_id BIGINT UNSIGNED NOT NULL,
+            delta DECIMAL(18,2) NOT NULL,
+            balance_after DECIMAL(18,2) NOT NULL,
+            reason VARCHAR(64) NOT NULL,
+            remark VARCHAR(255) DEFAULT NULL,
+            ref_type VARCHAR(32) DEFAULT NULL,
+            ref_id BIGINT UNSIGNED DEFAULT NULL,
+            ip VARCHAR(64) DEFAULT NULL,
+            ua VARCHAR(255) DEFAULT NULL,
+            created_at DATETIME NOT NULL,
+            PRIMARY KEY  (id),
+            KEY idx_user_time (user_id, created_at),
+            KEY idx_reason (reason)
+        ) {$charset_collate};";
+        dbDelta($sql_balance_log);
+
         // ---------- 2. 签到 ----------
         $sql_signin = "CREATE TABLE {$prefix}aksrt_signin_log (
             id BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,