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;