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/routes/history.py

@@ -0,0 +1,41 @@
+from fastapi import APIRouter, Depends
+from sqlalchemy.orm import Session
+from datetime import datetime, timezone
+
+from ..database import get_db
+from .. import models
+from ..dependencies import get_current_user
+from ..services.btc import get_btc_history_eur
+
+router = APIRouter()
+
+
+@router.get("/history")
+def get_history(
+    db: Session = Depends(get_db),
+    current_user: models.User = Depends(get_current_user),
+):
+    raw = get_btc_history_eur()
+
+    # Convert timestamps and deduplicate (keep last price per date)
+    seen = {}
+    for ts, price in raw:
+        date = datetime.fromtimestamp(ts / 1000, tz=timezone.utc).strftime("%Y-%m-%d")
+        seen[date] = round(price, 2)
+
+    prices = [{"date": d, "price": p} for d, p in seen.items()]
+
+    purchases_db = db.query(models.Purchase).filter(
+        models.Purchase.user_id == current_user.id
+    ).all()
+
+    purchases = [
+        {
+            "date": p.created_at.strftime("%Y-%m-%d"),
+            "amount_eur": round(p.amount_eur, 2),
+            "price_eur": round(p.price_eur, 2),
+        }
+        for p in purchases_db
+    ]
+
+    return {"prices": prices, "purchases": purchases}