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/docker-compose.yml

@@ -10,11 +10,12 @@ services:
     restart: unless-stopped
 
   frontend:
-    build: ./frontend
+    build:
+      context: ./frontend
+      args:
+        - REACT_APP_API_URL=http://localhost:8000
     ports:
-      - "3000:3000"
-    environment:
-      - REACT_APP_API_URL=http://localhost:8000
+      - "3001:3001"
     depends_on:
       - backend
     restart: unless-stopped