fix: poll for rendering_qc status and refresh timeline preview on completion

Fixes race condition where timeline preview never updated after AD VTT
re-render: useJob now polls every 5s during rendering_qc, and QCDetail
invalidates the edit-state query when the job transitions back to pending_qc.

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

frontend/src/hooks/useJob.ts

@@ -29,6 +29,10 @@ export function useJob(jobId: string) {
     enabled: !!jobId,
     staleTime: 30000, // 30 seconds
     refetchOnWindowFocus: false,
+    refetchInterval: (query) => {
+      const status = query.state.data?.status;
+      return status === 'rendering_qc' ? 5000 : false;
+    },
   });
 }
 

frontend/src/routes/admin/QCDetail.tsx

@@ -1,4 +1,5 @@
 import { useState, useEffect, useRef, useMemo } from 'react';
+import { useQueryClient } from '@tanstack/react-query';
 import { useParams, useNavigate } from 'react-router-dom';
 import { useJob, useApproveEnglish, useRejectJob, useJobVttContent, useUpdateJobVtt, useJobDownloads, useAdjustVttTiming, useUpdateTTSPreferences } from '../../hooks/useJob';
 import {
@@ -88,6 +89,7 @@ export function QCDetail() {
   const [adVttUploaded, setAdVttUploaded] = useState(false);
   const captionsFileInputRef = useRef<HTMLInputElement>(null);
   const adFileInputRef = useRef<HTMLInputElement>(null);
+  const prevJobStatusRef = useRef<string | undefined>(undefined);
   const [ttsPreferences, setTtsPreferences] = useState<TTSPreferences>({
     provider: 'gemini',
     default_voice: 'Kore',
@@ -119,6 +121,20 @@ export function QCDetail() {
     setLastDeletedAdCue(null);
   }, [selectedLanguage]);
 
+  // Refresh edit state when render completes (rendering_qc → pending_qc transition)
+  const queryClient = useQueryClient();
+  useEffect(() => {
+    if (
+      prevJobStatusRef.current === 'rendering_qc' &&
+      job?.status === 'pending_qc'
+    ) {
+      queryClient.invalidateQueries({
+        queryKey: ['jobs', id, 'accessible-video', selectedLanguage, 'edit-state'],
+      });
+    }
+    prevJobStatusRef.current = job?.status;
+  }, [job?.status, id, selectedLanguage, queryClient]);
+
   // Sync pending regenerations from server edit state
   useEffect(() => {
     if (editState?.tts_regeneration_queue) {