feat(J): WebSocket live-events + replace polling + fix ffmpeg turntable timeout

- fix(render): ffmpeg overlay=0:0 -> overlay=0:0:shortest=1 to prevent hang on finite PNG sequences
- feat(ws): add core/websocket.py ConnectionManager + Redis Pub/Sub subscriber loop
- feat(ws): add /api/ws WebSocket endpoint with JWT query-param auth in main.py
- feat(ws): emit render_complete/failed + cad_processing_complete events from step_tasks.py
- feat(ws): emit order_status_change events from orders router
- feat(ws): add beat_tasks.py broadcast_queue_status task (every 10s via Redis __broadcast__)
- feat(frontend): add useWebSocket hook with auto-reconnect (exponential backoff, 25s ping)
- feat(frontend): add WebSocketContext + WebSocketProvider wrapping App
- refactor(frontend): remove polling from WorkerActivity (was 5s/3s) + OrderDetail (was 5s)
- refactor(frontend): reduce polling in Layout (8s->60s) + NotificationCenter (15s->60s)
- docs: add ffmpeg shortest=1 + WebSocket JWT auth learnings to LEARNINGS.md

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

frontend/src/pages/OrderDetail.tsx

@@ -70,10 +70,6 @@ export default function OrderDetailPage() {
   const { data: order, isLoading } = useQuery({
     queryKey: ['order', id],
     queryFn: () => getOrder(id!),
-    refetchInterval: (query) => {
-      const status = query.state.data?.status
-      return status === 'processing' ? 5000 : false
-    },
   })
 
   const submitMut = useMutation({
@@ -169,7 +165,7 @@ export default function OrderDetailPage() {
   async function handleDownloadRenders() {
     setIsDownloading(true)
     try {
-      await downloadOrderRenders(id!, order.order_number)
+      await downloadOrderRenders(id!, order!.order_number)
     } catch (e: any) {
       toast.error(e.response?.data?.detail || 'Download failed')
     } finally {

frontend/src/pages/WorkerActivity.tsx

@@ -24,7 +24,6 @@ export default function WorkerActivityPage() {
   const { data, isLoading, dataUpdatedAt } = useQuery({
     queryKey: ['worker-activity'],
     queryFn: getWorkerActivity,
-    refetchInterval: 5000,
   })
 
   const reprocessMut = useMutation({
@@ -237,7 +236,7 @@ function QueuePanel() {
   const { data: queue, isLoading } = useQuery({
     queryKey: ['worker-queue'],
     queryFn: getQueueStatus,
-    refetchInterval: 3000,
+    refetchInterval: 10_000,
   })
 
   const purgeMut = useMutation({