feat(invite-code): one-time invite codes with rewards, usage log, import/export

- 一次性邀请码：批量生成（次数/有效期/备注/四类奖励）、原子消费防超发、
  开启注册验证时奖励延迟发放（账本幂等键）
- Zibll 兼容文本导入（points/balance/vip/vip_time/level_integral，分隔符可自定义，
  第三段标识存为备注）、CSV 全字段导出与可回流文本导出
- 使用记录表 aksrt_invite_code_usage + 后台使用者列；注册表单 400ms 防抖实时校验
- 注册门控三模式（close/open/must）设置链路（PHP schema + admin SPA schema）
- 契约测试覆盖建表/消费竞态/导入导出/备注全链路

src/Core/DBMigratorPart2.php

@@ -376,6 +376,40 @@ trait DBMigratorPart2 {
         ) ENGINE=InnoDB {$charset_collate};";
         dbDelta($sql_charge_card);
 
+        // ── one-time invite codes (registration gate + per-batch rewards) ──
+        $sql_invite_code = "CREATE TABLE {$prefix}aksrt_invite_code (
+            id              BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
+            code            VARCHAR(32) NOT NULL,
+            batch_id        VARCHAR(32) NOT NULL DEFAULT '',
+            remark          VARCHAR(191) NOT NULL DEFAULT '',
+            rewards         TEXT DEFAULT NULL,
+            max_uses        INT UNSIGNED NOT NULL DEFAULT 1,
+            used_count      INT UNSIGNED NOT NULL DEFAULT 0,
+            status          VARCHAR(20) NOT NULL DEFAULT 'active',
+            expires_at      DATETIME DEFAULT NULL,
+            created_by      BIGINT UNSIGNED NOT NULL DEFAULT 0,
+            created_at      DATETIME NOT NULL,
+            PRIMARY KEY  (id),
+            UNIQUE KEY uniq_invite_code (code),
+            KEY idx_batch (batch_id),
+            KEY idx_status (status),
+            KEY idx_expires (expires_at)
+        ) ENGINE=InnoDB {$charset_collate};";
+        dbDelta($sql_invite_code);
+
+        // ── invite-code usage log (who consumed which code) ──
+        $sql_invite_code_usage = "CREATE TABLE {$prefix}aksrt_invite_code_usage (
+            id              BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
+            code_id         BIGINT UNSIGNED NOT NULL,
+            user_id         BIGINT UNSIGNED NOT NULL,
+            used_at         DATETIME NOT NULL,
+            PRIMARY KEY  (id),
+            UNIQUE KEY uniq_invite_usage (code_id, user_id),
+            KEY idx_usage_user (user_id),
+            KEY idx_usage_code (code_id)
+        ) ENGINE=InnoDB {$charset_collate};";
+        dbDelta($sql_invite_code_usage);
+
         // ── canonical Ref attribution and permanent user binding ──
         $sql_affiliate_relation = "CREATE TABLE {$prefix}aksrt_affiliate_relation (
             id BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,