Add 1-year BTC daily price history chart

New GET /history endpoint fetches 365 days of BTC/EUR data from
CoinGecko, deduplicates by date, and joins the user's purchases.
BTCHistoryChart component renders the price line with orange dot
markers on purchase dates and a dashed cyan avg buy price line.
Tooltip shows purchase details on marked dates.

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

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

@@ -1,6 +1,19 @@
 import requests
 
 
+def get_btc_history_eur() -> list:
+    try:
+        resp = requests.get(
+            "https://api.coingecko.com/api/v3/coins/bitcoin/market_chart",
+            params={"vs_currency": "eur", "days": "365", "interval": "daily"},
+            timeout=15,
+        )
+        resp.raise_for_status()
+        return resp.json().get("prices", [])  # [[timestamp_ms, price], ...]
+    except Exception:
+        return []
+
+
 def get_btc_price_eur() -> float:
     try:
         resp = requests.get(