feat(ai_trade): SIGNAL_TTL_SECONDS env 추가 (F5 part 2)

신호 expires_at 계산용 TTL (default 300s). 환경별로 조정 가능.

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

ai_trade/tests/test_state_signals_lifecycle.py

@@ -48,3 +48,19 @@ def test_purge_expired_signals_removes_expired():
     assert "A" in state.signals
     assert "B" not in state.signals
     assert removed == 1
+
+
+# ----- SIGNAL_TTL_SECONDS env -----
+
+def test_signal_ttl_seconds_default(monkeypatch):
+    monkeypatch.delenv("SIGNAL_TTL_SECONDS", raising=False)
+    from ai_trade.config import Settings
+    s = Settings()
+    assert s.signal_ttl_seconds == 300
+
+
+def test_signal_ttl_seconds_env_override(monkeypatch):
+    monkeypatch.setenv("SIGNAL_TTL_SECONDS", "60")
+    from ai_trade.config import Settings
+    s = Settings()
+    assert s.signal_ttl_seconds == 60