feat: add portfolio optimization and congress tracking (TDD)

Portfolio optimization (3 endpoints):
- POST /portfolio/optimize - HRP optimal weights via scipy clustering
- POST /portfolio/correlation - pairwise correlation matrix
- POST /portfolio/risk-parity - inverse-volatility risk parity weights

Congress tracking (2 endpoints):
- GET /regulators/congress/trades - congress member stock trades
- GET /regulators/congress/bills?query= - search congress bills

Implementation:
- portfolio_service.py: HRP with scipy fallback to inverse-vol
- congress_service.py: multi-provider fallback pattern
- 51 new tests (14 portfolio unit, 20 portfolio route, 12 congress
  unit, 7 congress route)
- All 312 tests passing

tests/test_congress_service.py

@@ -0,0 +1,187 @@
+"""Tests for congress trading service (TDD - RED phase first)."""
+
+from unittest.mock import AsyncMock, MagicMock, patch
+
+import pytest
+
+
+# --- get_congress_trades ---
+
+
+@pytest.mark.asyncio
+async def test_get_congress_trades_happy_path():
+    """Returns list of trade dicts when OBB call succeeds."""
+    expected = [
+        {
+            "representative": "Nancy Pelosi",
+            "ticker": "NVDA",
+            "transaction_date": "2024-01-15",
+            "transaction_type": "Purchase",
+            "amount": "$1,000,001-$5,000,000",
+        }
+    ]
+
+    import congress_service
+
+    mock_fn = MagicMock()
+    with patch.object(congress_service, "_get_congress_fn", return_value=mock_fn), \
+         patch.object(congress_service, "_try_obb_call", new_callable=AsyncMock, return_value=expected):
+        result = await congress_service.get_congress_trades()
+
+    assert isinstance(result, list)
+    assert len(result) == 1
+    assert result[0]["representative"] == "Nancy Pelosi"
+
+
+@pytest.mark.asyncio
+async def test_get_congress_trades_returns_empty_when_fn_not_available():
+    """Returns empty list when OBB congress function is not available."""
+    import congress_service
+
+    with patch.object(congress_service, "_get_congress_fn", return_value=None):
+        result = await congress_service.get_congress_trades()
+
+    assert result == []
+
+
+@pytest.mark.asyncio
+async def test_get_congress_trades_returns_empty_on_all_provider_failures():
+    """Returns empty list when all providers fail (_try_obb_call returns None)."""
+    import congress_service
+
+    mock_fn = MagicMock()
+    with patch.object(congress_service, "_get_congress_fn", return_value=mock_fn), \
+         patch.object(congress_service, "_try_obb_call", new_callable=AsyncMock, return_value=None):
+        result = await congress_service.get_congress_trades()
+
+    assert result == []
+
+
+@pytest.mark.asyncio
+async def test_get_congress_trades_empty_list_result():
+    """Returns empty list when _try_obb_call returns empty list."""
+    import congress_service
+
+    mock_fn = MagicMock()
+    with patch.object(congress_service, "_get_congress_fn", return_value=mock_fn), \
+         patch.object(congress_service, "_try_obb_call", new_callable=AsyncMock, return_value=[]):
+        result = await congress_service.get_congress_trades()
+
+    assert result == []
+
+
+# --- _get_congress_fn ---
+
+
+def test_get_congress_fn_returns_none_when_attribute_missing():
+    """Returns None gracefully when obb.regulators.government_us is absent."""
+    import congress_service
+
+    mock_obb = MagicMock(spec=[])  # spec with no attributes
+    with patch.object(congress_service, "obb", mock_obb):
+        result = congress_service._get_congress_fn()
+
+    assert result is None
+
+
+def test_get_congress_fn_returns_callable_when_available():
+    """Returns the congress_trading callable when attribute exists."""
+    import congress_service
+
+    mock_fn = MagicMock()
+    mock_obb = MagicMock()
+    mock_obb.regulators.government_us.congress_trading = mock_fn
+
+    with patch.object(congress_service, "obb", mock_obb):
+        result = congress_service._get_congress_fn()
+
+    assert result is mock_fn
+
+
+# --- _try_obb_call ---
+
+
+@pytest.mark.asyncio
+async def test_try_obb_call_returns_list_on_success():
+    """_try_obb_call converts OBBject result to list via to_list."""
+    import congress_service
+
+    mock_result = MagicMock()
+    expected = [{"ticker": "AAPL"}]
+
+    with patch.object(congress_service, "to_list", return_value=expected), \
+         patch("congress_service.asyncio.to_thread", new_callable=AsyncMock, return_value=mock_result):
+        result = await congress_service._try_obb_call(MagicMock())
+
+    assert result == expected
+
+
+@pytest.mark.asyncio
+async def test_try_obb_call_returns_none_on_exception():
+    """_try_obb_call returns None when asyncio.to_thread raises."""
+    import congress_service
+
+    with patch("congress_service.asyncio.to_thread", new_callable=AsyncMock, side_effect=Exception("fail")):
+        result = await congress_service._try_obb_call(MagicMock())
+
+    assert result is None
+
+
+# --- search_congress_bills ---
+
+
+@pytest.mark.asyncio
+async def test_search_congress_bills_happy_path():
+    """Returns list of bill dicts when OBB call succeeds."""
+    expected = [
+        {"title": "Infrastructure Investment and Jobs Act", "bill_id": "HR3684"},
+        {"title": "Inflation Reduction Act", "bill_id": "HR5376"},
+    ]
+
+    import congress_service
+
+    mock_fn = MagicMock()
+    with patch.object(congress_service, "_get_congress_fn", return_value=mock_fn), \
+         patch.object(congress_service, "_try_obb_call", new_callable=AsyncMock, return_value=expected):
+        result = await congress_service.search_congress_bills("infrastructure")
+
+    assert isinstance(result, list)
+    assert len(result) == 2
+    assert result[0]["bill_id"] == "HR3684"
+
+
+@pytest.mark.asyncio
+async def test_search_congress_bills_returns_empty_when_fn_not_available():
+    """Returns empty list when OBB function is not available."""
+    import congress_service
+
+    with patch.object(congress_service, "_get_congress_fn", return_value=None):
+        result = await congress_service.search_congress_bills("taxes")
+
+    assert result == []
+
+
+@pytest.mark.asyncio
+async def test_search_congress_bills_returns_empty_on_failure():
+    """Returns empty list when all providers fail."""
+    import congress_service
+
+    mock_fn = MagicMock()
+    with patch.object(congress_service, "_get_congress_fn", return_value=mock_fn), \
+         patch.object(congress_service, "_try_obb_call", new_callable=AsyncMock, return_value=None):
+        result = await congress_service.search_congress_bills("taxes")
+
+    assert result == []
+
+
+@pytest.mark.asyncio
+async def test_search_congress_bills_empty_results():
+    """Returns empty list when _try_obb_call returns empty list."""
+    import congress_service
+
+    mock_fn = MagicMock()
+    with patch.object(congress_service, "_get_congress_fn", return_value=mock_fn), \
+         patch.object(congress_service, "_try_obb_call", new_callable=AsyncMock, return_value=[]):
+        result = await congress_service.search_congress_bills("nonexistent")
+
+    assert result == []