feat: add Bitfinex price fetcher for BTC/EUR

backend/price_fetcher.py

@@ -0,0 +1,38 @@
+"""Bitfinex price fetcher for BTC/EUR pair."""
+
+from datetime import UTC, datetime
+
+import httpx
+
+BITFINEX_TICKER_URL = "https://api-pub.bitfinex.com/v2/ticker/tBTCEUR"
+LAST_PRICE_INDEX = 6
+
+
+async def fetch_btc_eur_price() -> tuple[float, datetime]:
+    """
+    Fetch the current BTC/EUR price from Bitfinex.
+
+    Returns:
+        Tuple of (price, timestamp) where timestamp is the current UTC time.
+
+    Raises:
+        httpx.HTTPStatusError: If the API returns a non-2xx status code.
+        httpx.RequestError: If a network error occurs.
+        ValueError: If the response format is unexpected.
+    """
+    async with httpx.AsyncClient() as client:
+        response = await client.get(BITFINEX_TICKER_URL)
+        response.raise_for_status()
+
+        data = response.json()
+
+        # Bitfinex ticker response is an array:
+        # [BID, BID_SIZE, ASK, ASK_SIZE, DAILY_CHANGE, DAILY_CHANGE_RELATIVE,
+        #  LAST_PRICE, VOLUME, HIGH, LOW]
+        if not isinstance(data, list) or len(data) <= LAST_PRICE_INDEX:
+            raise ValueError(f"Unexpected response format: {data}")
+
+        price = float(data[LAST_PRICE_INDEX])
+        timestamp = datetime.now(UTC)
+
+        return price, timestamp