Extract price logic to PriceService

- Create PriceService with get_recent_prices() and fetch_and_store_price()
- Update routes/audit.py to use PriceService instead of direct queries
- Use PriceHistoryMapper consistently
- Update test to patch services.price.fetch_btc_eur_price

backend/tests/test_price_history.py

@@ -280,7 +280,7 @@ class TestManualFetch:
             existing_id = existing.id
 
         # Mock fetch_btc_eur_price to return the same timestamp
-        with patch("routes.audit.fetch_btc_eur_price") as mock_fetch:
+        with patch("services.price.fetch_btc_eur_price") as mock_fetch:
             mock_fetch.return_value = (95000.0, fixed_timestamp)
 
             async with client_factory.create(cookies=admin_user["cookies"]) as authed: