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/contexts/WebSocketContext.tsx

@@ -0,0 +1,62 @@
+/**
+ * WebSocketContext — global WebSocket provider.
+ *
+ * Wraps the app with a single WebSocket connection. On incoming events it
+ * invalidates the relevant React Query caches so all subscribers refresh.
+ */
+import { createContext, useContext, useCallback } from 'react'
+import { useQueryClient } from '@tanstack/react-query'
+import { useWebSocket, type WSEvent } from '../hooks/useWebSocket'
+
+const WebSocketContext = createContext<null>(null)
+
+export function WebSocketProvider({ children }: { children: React.ReactNode }) {
+  const qc = useQueryClient()
+
+  const onEvent = useCallback(
+    (event: WSEvent) => {
+      switch (event.type) {
+        case 'render_complete':
+        case 'render_failed':
+          qc.invalidateQueries({ queryKey: ['orders'] })
+          if (event.order_id) {
+            qc.invalidateQueries({ queryKey: ['order', event.order_id as string] })
+          }
+          qc.invalidateQueries({ queryKey: ['worker-activity'] })
+          break
+
+        case 'cad_processing_complete':
+          qc.invalidateQueries({ queryKey: ['worker-activity'] })
+          break
+
+        case 'order_status_change':
+          qc.invalidateQueries({ queryKey: ['orders'] })
+          if (event.order_id) {
+            qc.invalidateQueries({ queryKey: ['order', event.order_id as string] })
+          }
+          break
+
+        case 'queue_update':
+          qc.setQueryData(['queue-status'], event.depths)
+          break
+
+        default:
+          break
+      }
+    },
+    [qc],
+  )
+
+  useWebSocket({ onEvent })
+
+  return (
+    <WebSocketContext.Provider value={null}>
+      {children}
+    </WebSocketContext.Provider>
+  )
+}
+
+// eslint-disable-next-line @typescript-eslint/no-unused-vars
+export function useWebSocketContext() {
+  return useContext(WebSocketContext)
+}