fix: keep Alpaca price as primary (includes after-hours), add polygon_price field

Alpaca's current_price reflects extended hours trading which is more
current than Polygon's regular session close. Keep it as the display
price. Add polygon_price as a reference field in the API response.

frontend/src/api/hooks.ts

@@ -389,6 +389,7 @@ export interface Position {
   quantity: number;
   avg_entry_price: number;
   current_price: number | null;
+  polygon_price: number | null;
   unrealized_pnl: number | null;
   realized_pnl: number | null;
   updated_at: string;

services/api/app.py

@@ -1201,7 +1201,8 @@ async def list_positions(
                FROM positions p ORDER BY p.ticker""",
         )
 
-    # Build a price map from the latest Polygon bars
+    # Enrich with latest Polygon close for comparison.
+    # Use whichever price is more recent: broker sync or Polygon bar.
     tickers = list({r["ticker"] for r in rows})
     price_map: dict[str, float] = {}
     if tickers:
@@ -1218,11 +1219,7 @@ async def list_positions(
     for r in rows:
         d = _row_to_dict(r)
         polygon_price = price_map.get(d["ticker"])
-        if polygon_price:
+        d["polygon_price"] = polygon_price
-            d["current_price"] = polygon_price
-            qty = d.get("quantity", 0) or 0
-            entry = d.get("avg_entry_price", 0) or 0
-            d["unrealized_pnl"] = round(qty * (polygon_price - entry), 2)
         results.append(d)
     return results