refactor(referral): prune dead code and formalize ops list header

- remove unused AuthReader::invitation_mode and ReferralAttribution
  helpers (get_or_create_code / validate_ref); contract test now
  asserts the canonical invit_code_s reader in CreditsRuntime
- drop legacy aksrt_invite table from uninstall cleanup list
- track referral code creation time (aksrt_referral_code_created)
  and surface it in the referral ops REST list
- formalize ops list headers (归属用户/推广码/启用状态/推广用户数/
  有效付费用户数/创建时间) and right-align count columns

src/Settings/AuthReader.php

@@ -184,12 +184,6 @@ trait AuthReader {
         return (bool) self::get_value('user_signup_no_repas', true);
     }
 
-    /** 推广码在注册表单中的唯一显示模式。 */
-    public static function invitation_mode(): string {
-        $mode = (string) self::get_value('invit_code_s', 'close');
-        return in_array($mode, ['close', 'open', 'must'], true) ? $mode : 'close';
-    }
-
     /** 自动弹窗配置。 */
     public static function sign_modal_auto(): array {
         return [