Guard against overlapping reorder requests; drop dead pivot guards

- Ignore further drops while a reorder request is in flight, so rapid
  successive drags can't race and persist a stale order (the response that
  lands last would otherwise win).
- Remove the `$record->pivot` null guards in applyDeferredRelationOrder:
  records from a sortable belongsToMany/morphToMany/morphedByMany relation
  always carry a hydrated pivot, verified in both new-parent and
  existing-parent deferred/orphan modes, so the guards were unreachable.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: LukeTowers

modules/backend/behaviors/RelationController.php

@@ -622,12 +622,13 @@ protected function applyDeferredRelationOrder($records)
         }
 
         foreach ($records as $record) {
-            if ($record->pivot && array_key_exists($record->getKey(), $map)) {
+            if (array_key_exists($record->getKey(), $map)) {
                 $record->pivot->{$column} = $map[$record->getKey()];
             }
         }
 
         return $records->sortBy(function ($record) use ($column) {
+            // Deferred records not yet assigned an order sort to the end.
             return $record->pivot->{$column} ?? PHP_INT_MAX;
         })->values();
     }