feat(upload): pending lifecycle for upload-on-select images

Mark front-end uploads (comment/ticket/report) as pending, return the
attachment id, discard own pending uploads on remove via
DELETE /upload/pending/{id}, claim referenced URLs on successful submit,
and purge unclaimed uploads older than 24h with an hourly cron.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/Admin/UserControllerReportOperations.php

@@ -450,6 +450,8 @@ trait UserControllerReportOperations {
         );
 
         if ($report_id > 0) {
+            // 举证图片以 URL 拼入举报内容，提交成功即认领。
+            \AKSRT\Attachment\PendingUploads::claim_text($reason, $reporter_id);
             return $this->success([
                 'message' => __('举报已提交，我们会尽快处理', 'aksrt'),
                 'data'    => ['report_id' => $report_id],