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/worker.py

@@ -14,7 +14,7 @@ from pgqueuer.queries import Queries
 
 from database import ASYNCPG_DATABASE_URL
 from jobs import JOB_FETCH_BITCOIN_PRICE, JOB_RANDOM_NUMBER
-from price_fetcher import fetch_btc_eur_price
+from price_fetcher import PAIR_BTC_EUR, SOURCE_BITFINEX, fetch_btc_eur_price
 
 logging.basicConfig(
     level=logging.INFO,
@@ -116,8 +116,8 @@ async def process_bitcoin_price_job(db_pool: asyncpg.Pool) -> None:
                 VALUES ($1, $2, $3, $4, NOW())
                 ON CONFLICT (source, pair, timestamp) DO NOTHING
                 """,
-                "bitfinex",
-                "BTC/EUR",
+                SOURCE_BITFINEX,
+                PAIR_BTC_EUR,
                 price,
                 timestamp,
             )