Add BTC candlestick chart with local OHLC storage

- Store daily BTC OHLC candles in SQLite to avoid hitting CoinGecko on every load
- Seed with 30 days of daily candles on first boot (free tier gives daily granularity for days<=30)
- Auto-detect and replace coarse legacy candle data on startup
- Daily refresh adds new candles on each container restart
- New GET /candles endpoint (supports ?days=365 and ?days=all)
- Switch BTCHistoryChart to BTCCandlestickChart using lightweight-charts (TradingView)
- Purchase markers with nearest-candle matching and multi-purchase merging
- Fullscreen mode showing all stored candles
- Fix frontend port to 3001 and pass REACT_APP_API_URL as build arg

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

btc-portfolio/backend/app/routes/candles.py

@@ -0,0 +1,53 @@
+from fastapi import APIRouter, Depends, Query
+from sqlalchemy.orm import Session
+
+from ..database import get_db
+from .. import models
+from ..dependencies import get_current_user
+
+router = APIRouter()
+
+
+@router.get("/candles")
+def get_candles(
+    days: str = Query(default="365"),
+    db: Session = Depends(get_db),
+    current_user: models.User = Depends(get_current_user),
+):
+    query = db.query(models.OHLCCandle).order_by(models.OHLCCandle.date.asc())
+
+    if days != "all":
+        try:
+            limit = int(days)
+        except ValueError:
+            limit = 365
+        # Fetch the most recent `limit` rows
+        total = query.count()
+        if total > limit:
+            query = query.offset(total - limit)
+
+    candles_db = query.all()
+    candles = [
+        {
+            "date": c.date,
+            "open": round(c.open, 2),
+            "high": round(c.high, 2),
+            "low": round(c.low, 2),
+            "close": round(c.close, 2),
+        }
+        for c in candles_db
+    ]
+
+    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 {"candles": candles, "purchases": purchases}