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>

backend/app/tasks/celery_app.py

@@ -1,4 +1,5 @@
 from celery import Celery
+from celery.schedules import crontab
 from app.config import settings
 
 celery_app = Celery(
@@ -8,6 +9,7 @@ celery_app = Celery(
     include=[
         "app.tasks.step_tasks",
         "app.tasks.ai_tasks",
+        "app.tasks.beat_tasks",
         "app.domains.rendering.tasks",
         "app.domains.products.tasks",
         "app.domains.imports.tasks",
@@ -23,7 +25,13 @@ celery_app.conf.update(
     task_routes={
         "app.tasks.step_tasks.*": {"queue": "step_processing"},
         "app.tasks.ai_tasks.*": {"queue": "ai_validation"},
+        "app.tasks.beat_tasks.*": {"queue": "step_processing"},
         "app.domains.rendering.tasks.*": {"queue": "thumbnail_rendering"},
     },
-    beat_schedule={},
+    beat_schedule={
+        "broadcast-queue-status-every-10s": {
+            "task": "app.tasks.beat_tasks.broadcast_queue_status",
+            "schedule": 10.0,  # every 10 seconds
+        },
+    },
 )