feat(ai_trade): NAS Redis heartbeat (trader market_open/closed)

- ai_trade/heartbeat.py: build_trader_payload() + heartbeat_loop() 자체 미니 헬퍼
  (Windows 호스트 실행이라 _shared import 경로 달라 독립 구현, 계약은 동일)
- ai_trade/main.py: lifespan에 hb_task spawn + shutdown 시 cancel
  state_fn = scheduler._is_market_day & _is_polling_window(KST now) 조합
  signals = len(state.signals) 실시간 주입
- requirements.txt: redis>=5.0 추가
- ai_trade/tests/test_heartbeat.py: build_trader_payload 3케이스 TDD 검증

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019LV86jBozkNhSFXJA412fq

requirements.txt

@@ -7,6 +7,7 @@ pytest>=8.0
 pytest-asyncio>=0.23
 respx>=0.21
 websockets>=12
+redis>=5.0
 # Phase 3b dependencies (Chronos-2 + ML)
 transformers>=4.40
 chronos-forecasting>=1.4