fix: add KR_FALLBACK_STOCKS for domestic scanner when ranking API returns empty (#153)

KIS VTS (paper trading) does not return data from the domestic
volume-rank API. The smart scanner had no fallback for KR market,
causing permanent "No candidates" and zero trades all day.

- src/config.py: add KR_FALLBACK_STOCKS with 10 major KRX stocks
  (삼성전자, SK하이닉스, NAVER, 현대차, 셀트리온, LG화학, 카카오,
   삼성SDI, 삼성바이오로직스, 기아)
- src/main.py: pass KR fallback stocks to scanner.scan() for domestic
  markets (mirrors the overseas build_overseas_symbol_universe pattern)
- tests/test_main.py: add TestKrFallbackStocksConfig (3 tests)

Closes #153

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_main.py

@@ -1678,3 +1678,43 @@ def test_start_dashboard_server_enabled_starts_thread() -> None:
     assert thread == mock_thread
     mock_thread_cls.assert_called_once()
     mock_thread.start.assert_called_once()
+
+
+# ---------------------------------------------------------------------------
+# KR fallback stocks config (issue #153)
+# ---------------------------------------------------------------------------
+
+
+class TestKrFallbackStocksConfig:
+    """Test KR_FALLBACK_STOCKS default value and parsing."""
+
+    def test_default_contains_samsung(self) -> None:
+        settings = Settings(
+            KIS_APP_KEY="k",
+            KIS_APP_SECRET="s",
+            KIS_ACCOUNT_NO="12345678-01",
+            GEMINI_API_KEY="g",
+        )
+        codes = [c.strip() for c in settings.KR_FALLBACK_STOCKS.split(",") if c.strip()]
+        assert "005930" in codes  # 삼성전자
+
+    def test_default_has_ten_codes(self) -> None:
+        settings = Settings(
+            KIS_APP_KEY="k",
+            KIS_APP_SECRET="s",
+            KIS_ACCOUNT_NO="12345678-01",
+            GEMINI_API_KEY="g",
+        )
+        codes = [c.strip() for c in settings.KR_FALLBACK_STOCKS.split(",") if c.strip()]
+        assert len(codes) == 10
+
+    def test_env_override(self, monkeypatch: pytest.MonkeyPatch) -> None:
+        monkeypatch.setenv("KR_FALLBACK_STOCKS", "005930,000660")
+        settings = Settings(
+            KIS_APP_KEY="k",
+            KIS_APP_SECRET="s",
+            KIS_ACCOUNT_NO="12345678-01",
+            GEMINI_API_KEY="g",
+        )
+        codes = [c.strip() for c in settings.KR_FALLBACK_STOCKS.split(",") if c.strip()]
+        assert codes == ["005930", "000660"]