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/Settings/CreditsRuntime.php

@@ -91,13 +91,18 @@ class CreditsRuntime {
     /**
      * Shared ref-code settings.
      *
-     * @return array{code_length:int,window_days:int}
+     * @return array{code_length:int,window_days:int,attribution_model:string}
      */
     public static function get_referral_settings(): array {
         $all = Settings::get_all();
+        $attribution = sanitize_key((string) ($all['referral_attribution_model'] ?? 'first_touch'));
+        if (!in_array($attribution, ['first_touch', 'last_touch'], true)) {
+            $attribution = 'first_touch';
+        }
         return [
             'code_length' => max(4, min(12, (int) ($all['referral_code_length'] ?? 12))),
             'window_days' => max(1, min(365, (int) ($all['referral_window_days'] ?? 30))),
+            'attribution_model' => $attribution,
         ];
     }
 
@@ -145,6 +150,32 @@ class CreditsRuntime {
         ];
     }
 
+    /**
+     * @return array{enabled:bool,register_field:string,open_desc:string,required_desc:string}
+     */
+    public static function get_invite_code_registration_settings(): array {
+        $all = Settings::get_all();
+        $mode = sanitize_key((string) ($all['invite_code_s'] ?? 'close'));
+        if (!in_array($mode, ['close', 'open', 'must'], true)) {
+            $mode = 'close';
+        }
+        $open_desc = trim((string) ($all['invite_code_open_desc'] ?? ''));
+        if ($open_desc === '') {
+            $open_desc = __('请输入邀请码，注册成功后奖励将自动发放', 'aksrt');
+        }
+        $required_desc = trim((string) ($all['invite_code_must_desc'] ?? ''));
+        if ($required_desc === '') {
+            $required_desc = __('<div>本站需凭邀请码注册</div><div>获取邀请码请与客服联系</div>', 'aksrt');
+        }
+
+        return [
+            'enabled'        => $mode !== 'close',
+            'register_field' => $mode === 'must' ? 'required' : ($mode === 'open' ? 'optional' : 'hidden'),
+            'open_desc'      => $open_desc,
+            'required_desc'  => $required_desc,
+        ];
+    }
+
     /**
      * 积分功能是否启用。
      * 仅看积分系统开关；不再与 cms_features.download_credit 联合（该开关表示积分购买模式，非按次扣费）。