Cache last known BTC price and show stale warning in UI

When the CoinGecko API fails, fall back to the last successful price
instead of 0.0, and surface a warning indicator on the price card.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

btc-portfolio/backend/app/services/btc.py

@@ -54,7 +54,12 @@ def aggregate_to_daily(raw: list) -> dict:
     return by_date
 
 
-def get_btc_price_eur() -> float:
+_last_known_price: float = 0.0
+
+
+def get_btc_price_eur() -> tuple[float, bool]:
+    """Returns (price, is_cached). is_cached=True when using a stale fallback."""
+    global _last_known_price
     try:
         resp = requests.get(
             "https://api.coingecko.com/api/v3/simple/price",
@@ -62,7 +67,9 @@ def get_btc_price_eur() -> float:
             timeout=10,
         )
         resp.raise_for_status()
-        return float(resp.json()["bitcoin"]["eur"])
+        price = float(resp.json()["bitcoin"]["eur"])
+        _last_known_price = price
+        return price, False
     except Exception as e:
         logger.error(f"Failed to fetch BTC price: {e}")
-        return 0.0
+        return _last_known_price, True