feat(signal_v2): rate_limit + 3 unit tests

SignalDedup: 24h-rolling duplicate signal blocker. SQLite WAL +
busy_timeout=120000 standard fix (reference_sqlite_concurrency.md
pattern). PK (ticker, action) with UPSERT. Phase 4 (signal generator)
will call is_recent() before sending + record() after sending.

3 unit tests pass, total 17 signal_v2.

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

signal_v2/tests/test_rate_limit.py

@@ -0,0 +1,34 @@
+"""Tests for SignalDedup."""
+from datetime import datetime, timedelta
+from zoneinfo import ZoneInfo
+
+from signal_v2.rate_limit import SignalDedup
+
+KST = ZoneInfo("Asia/Seoul")
+
+
+def test_is_recent_returns_false_for_new_ticker_action(tmp_dedup_db):
+    dedup = SignalDedup(tmp_dedup_db)
+    assert dedup.is_recent("005930", "buy") is False
+
+
+def test_is_recent_returns_true_within_24h(tmp_dedup_db):
+    dedup = SignalDedup(tmp_dedup_db)
+    dedup.record("005930", "buy", confidence=0.82)
+    assert dedup.is_recent("005930", "buy") is True
+
+
+def test_is_recent_returns_false_after_24h(tmp_dedup_db, monkeypatch):
+    dedup = SignalDedup(tmp_dedup_db)
+    # Record with a timestamp 25 hours ago
+    now = datetime.now(KST)
+    fake_now = now - timedelta(hours=25)
+    monkeypatch.setattr(
+        "signal_v2.rate_limit._now_iso", lambda: fake_now.isoformat()
+    )
+    dedup.record("005930", "buy", confidence=0.82)
+    # Reset to real now for is_recent check
+    monkeypatch.setattr(
+        "signal_v2.rate_limit._now_iso", lambda: now.isoformat()
+    )
+    assert dedup.is_recent("005930", "buy", within_hours=24) is False