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/Payment/ReferralAttribution.php

@@ -14,6 +14,8 @@ final class ReferralAttribution {
 	private const COOKIE = 'aksrt_referral';
 	private const META_CODE = 'aksrt_referral_code';
 	private const META_STATUS = 'aksrt_referral_code_status';
+	/** 推广码生成时间（新生成的码才有；历史码为空，后台列表显示 —）。 */
+	private const META_CREATED = 'aksrt_referral_code_created';
 	private const STATUS_ACTIVE = 'active';
 	private const STATUS_DISABLED = 'disabled';
 
@@ -258,14 +260,6 @@ final class ReferralAttribution {
 		];
 	}
 
-	public function get_or_create_code(int $user_id): string {
-		return (string) ($this->create_link($user_id)['ref'] ?? '');
-	}
-
-	public function validate_ref(string $ref): bool {
-		return $this->resolve_promoter($ref) > 0;
-	}
-
 	public function resolve(int $user_id = 0): int {
 		$user_id = $user_id > 0 ? $user_id : get_current_user_id();
 		if ($user_id <= 0) {
@@ -482,9 +476,10 @@ final class ReferralAttribution {
 
 		$where_sql = implode(' AND ', $where);
 		$joins = $wpdb->prepare(
-			"LEFT JOIN {$wpdb->usermeta} code ON code.user_id=u.ID AND code.meta_key=%s LEFT JOIN {$wpdb->usermeta} state ON state.user_id=u.ID AND state.meta_key=%s",
+			"LEFT JOIN {$wpdb->usermeta} code ON code.user_id=u.ID AND code.meta_key=%s LEFT JOIN {$wpdb->usermeta} state ON state.user_id=u.ID AND state.meta_key=%s LEFT JOIN {$wpdb->usermeta} created ON created.user_id=u.ID AND created.meta_key=%s",
 			self::META_CODE,
-			self::META_STATUS
+			self::META_STATUS,
+			self::META_CREATED
 		);
 		$count_sql = "SELECT COUNT(DISTINCT u.ID) FROM {$wpdb->users} u {$joins} WHERE {$where_sql}";
 		$total = (int) ($params
@@ -495,7 +490,7 @@ final class ReferralAttribution {
 		$ledger_table = \AKSRT\Core\DBMigrator::table('commission_ledger');
 		$stats_joins = "LEFT JOIN (SELECT promoter_id, COUNT(*) AS bound_count FROM {$relation_table} WHERE status='bound' GROUP BY promoter_id) relation_stats ON relation_stats.promoter_id=u.ID "
 			. "LEFT JOIN (SELECT promoter_id, COUNT(DISTINCT source_user_id) AS effective_payers FROM {$ledger_table} WHERE original_amount > 0 GROUP BY promoter_id) payer_stats ON payer_stats.promoter_id=u.ID";
-		$list_sql = "SELECT DISTINCT u.ID, u.user_login, u.display_name, u.user_email, code.meta_value AS ref, state.meta_value AS ref_status, "
+		$list_sql = "SELECT DISTINCT u.ID, u.user_login, u.display_name, u.user_email, code.meta_value AS ref, state.meta_value AS ref_status, created.meta_value AS created_at, "
 			. "COALESCE(relation_stats.bound_count, 0) AS bound_count, COALESCE(payer_stats.effective_payers, 0) AS effective_payers "
 			. "FROM {$wpdb->users} u {$joins} {$stats_joins} WHERE {$where_sql} ORDER BY u.ID DESC LIMIT %d OFFSET %d";
 		$list_params = array_merge($params, [$per_page, ($page - 1) * $per_page]);
@@ -514,6 +509,7 @@ final class ReferralAttribution {
 				],
 				'ref' => $ref,
 				'status' => $ref === '' ? 'missing' : ((string) ($row['ref_status'] ?? '') === self::STATUS_DISABLED ? self::STATUS_DISABLED : self::STATUS_ACTIVE),
+				'created_at' => (string) ($row['created_at'] ?? ''),
 				'bound_count' => (int) ($row['bound_count'] ?? 0),
 				'effective_payers' => (int) ($row['effective_payers'] ?? 0),
 			];
@@ -553,6 +549,7 @@ final class ReferralAttribution {
 			if (empty($owners)) {
 				update_user_meta($user_id, self::META_CODE, $code);
 				update_user_meta($user_id, self::META_STATUS, self::STATUS_ACTIVE);
+				update_user_meta($user_id, self::META_CREATED, current_time('mysql'));
 				$updated = $this->code_record($user_id);
 				if ($updated['code'] === $code && $updated['status'] === self::STATUS_ACTIVE) {
 					return $code;