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/services/render_blender.py

@@ -504,7 +504,7 @@ def render_turntable_to_file(
             "-framerate", str(fps),
             "-i", str(frames_dir / "frame_%04d.png"),
             "-f", "lavfi", "-i", f"color=c=0x{hex_color}:size={width}x{height}:rate={fps}",
-            "-filter_complex", "[1:v][0:v]overlay=0:0",
+            "-filter_complex", "[1:v][0:v]overlay=0:0:shortest=1",
             "-vcodec", "libx264",
             "-pix_fmt", "yuv420p",
             "-crf", "18",