feat(scheduler): delete orphaned jobs and block editing of unregistered jobs

Adds deleteOrphanedJobs() to clean up stale job_keys during ensureSystemJobs, and prevents editing jobs that are no longer registered in the runtime registry.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/Service/Scheduler/ScheduledJobService.php

@@ -42,6 +42,13 @@ class ScheduledJobService
             }
             $this->scheduledJobRepository->updateJobMeta((int) $existing['id'], $normalized);
         }
+
+        $this->scheduledJobRepository->deleteOrphanedJobs(array_keys($definitions));
+    }
+
+    public function isRegistered(string $jobKey): bool
+    {
+        return $this->scheduledJobRegistry->get($jobKey) !== null;
     }
 
     public function listPaged(array $filters): array