fix: Remove agreed_price from price API response

The agreed_price depends on trade direction (buy/sell) and must be
calculated on the frontend. Returning a buy-side-only agreed_price
from the API was misleading and unused.

Frontend already calculates the direction-aware price correctly.

backend/tests/test_exchange.py

@@ -98,8 +98,7 @@ class TestExchangePriceEndpoint:
         assert "config" in data
         assert data["price"]["market_price"] == 20000.0
         assert data["price"]["premium_percentage"] == 5
-        # Agreed price should be market * 1.05 (5% premium)
-        assert data["price"]["agreed_price"] == pytest.approx(21000.0, rel=0.001)
+        # Note: agreed_price is calculated on frontend based on direction (buy/sell)
 
     @pytest.mark.asyncio
     async def test_admin_cannot_get_price(self, client_factory, admin_user):