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>

functions.php

@@ -313,6 +313,9 @@ if (class_exists('\AKSRT\Core\FrontSubmit')) {
 if (class_exists('\AKSRT\Attachment\UploadLimits')) {
     \AKSRT\Attachment\UploadLimits::get_instance()->init();
 }
+if (class_exists('\AKSRT\Attachment\PendingUploads')) {
+    \AKSRT\Attachment\PendingUploads::get_instance()->init();
+}
 if (class_exists('\AKSRT\Payment\Order')) {
     \AKSRT\Payment\Order::get_instance()->init();
 }