feat(ai_trade): emit signal에 cycle_id + expires_at 부착 (F5 part 3)

- generate_signals 진입에서 state.signal_cycle_id += 1 (emit 여부 무관 증가)
- _build_buy_signal/_build_sell_signal에 cycle_id + expires_at 필드 추가
- expires_at = as_of + settings.signal_ttl_seconds (default 300s)
- 매수/매도 양쪽 로그에 cycle=N 추가

기존 test_poll_loop_calls_generate_signals_after_cycle의 settings MagicMock에
signal_ttl_seconds=300 명시 (timedelta가 MagicMock 받으면 TypeError).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ai_trade/tests/test_pull_worker.py

@@ -122,6 +122,7 @@ def test_poll_loop_calls_generate_signals_after_cycle(monkeypatch):
     settings.asking_bid_ratio_threshold = 0.6
     settings.confidence_threshold = 0.7
     settings.min_momentum_for_buy = "strong_up"
+    settings.signal_ttl_seconds = 300
 
     generate_signals(state, dedup, settings)