refactor: extract 'bitfinex' and 'BTC/EUR' magic strings to constants

Add SOURCE_BITFINEX and PAIR_BTC_EUR constants in price_fetcher.py and
use them consistently in routes/audit.py, worker.py, and tests.

backend/price_fetcher.py

@@ -7,6 +7,10 @@ import httpx
 BITFINEX_TICKER_URL = "https://api-pub.bitfinex.com/v2/ticker/tBTCEUR"
 LAST_PRICE_INDEX = 6
 
+# Constants for price history records
+SOURCE_BITFINEX = "bitfinex"
+PAIR_BTC_EUR = "BTC/EUR"
+
 
 async def fetch_btc_eur_price() -> tuple[float, datetime]:
     """