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/routes/audit.py

@@ -23,7 +23,7 @@ from pagination import (
     calculate_total_pages,
     create_paginated_response,
 )
-from price_fetcher import fetch_btc_eur_price
+from price_fetcher import PAIR_BTC_EUR, SOURCE_BITFINEX, fetch_btc_eur_price
 from schemas import (
     CounterRecordResponse,
     PaginatedCounterRecords,
@@ -194,8 +194,8 @@ async def fetch_price_now(
     price, timestamp = await fetch_btc_eur_price()
 
     record = PriceHistory(
-        source="bitfinex",
-        pair="BTC/EUR",
+        source=SOURCE_BITFINEX,
+        pair=PAIR_BTC_EUR,
         price=price,
         timestamp=timestamp,
     )