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

congress_service.py

@@ -0,0 +1,68 @@
+"""Congress trading data: member trades and bill search."""
+
+import asyncio
+import logging
+from typing import Any
+
+from openbb import obb
+
+from obb_utils import to_list
+
+logger = logging.getLogger(__name__)
+
+
+async def _try_obb_call(fn, *args, **kwargs) -> list[dict[str, Any]] | None:
+    """Attempt a single OBB call and return to_list result, or None on failure."""
+    try:
+        result = await asyncio.to_thread(fn, *args, **kwargs)
+        return to_list(result)
+    except Exception as exc:
+        logger.debug("OBB call failed: %s", exc)
+        return None
+
+
+def _get_congress_fn():
+    """Resolve the congress trading OBB function safely."""
+    try:
+        return obb.regulators.government_us.congress_trading
+    except AttributeError:
+        logger.debug("obb.regulators.government_us.congress_trading not available")
+        return None
+
+
+async def get_congress_trades() -> list[dict[str, Any]]:
+    """Get recent US congress member stock trades.
+
+    Returns an empty list if the data provider is unavailable.
+    """
+    fn = _get_congress_fn()
+    if fn is None:
+        return []
+
+    providers = ["quiverquant", "fmp"]
+    for provider in providers:
+        data = await _try_obb_call(fn, provider=provider)
+        if data is not None:
+            return data
+
+    logger.warning("All congress trades providers failed")
+    return []
+
+
+async def search_congress_bills(query: str) -> list[dict[str, Any]]:
+    """Search US congress bills by keyword.
+
+    Returns an empty list if the data provider is unavailable.
+    """
+    fn = _get_congress_fn()
+    if fn is None:
+        return []
+
+    providers = ["quiverquant", "fmp"]
+    for provider in providers:
+        data = await _try_obb_call(fn, query, provider=provider)
+        if data is not None:
+            return data
+
+    logger.warning("All congress bills providers failed for query: %s", query)
+    return []