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/main.py

@@ -1,8 +1,8 @@
 from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 
-from .database import engine, Base
-from .routes import users, purchases, stats, history
+from .database import engine, Base, SessionLocal
+from .routes import users, purchases, stats, history, candles
 
 Base.metadata.create_all(bind=engine)
 
@@ -10,7 +10,7 @@ app = FastAPI(title="BTC Portfolio API")
 
 app.add_middleware(
     CORSMiddleware,
-    allow_origins=["http://localhost:3000"],
+    allow_origins=["http://localhost:3000", "http://localhost:3001"],
     allow_credentials=True,
     allow_methods=["*"],
     allow_headers=["*"],
@@ -20,6 +20,17 @@ app.include_router(users.router)
 app.include_router(purchases.router)
 app.include_router(stats.router)
 app.include_router(history.router)
+app.include_router(candles.router)
+
+
+@app.on_event("startup")
+def startup():
+    from .services.candles import refresh_latest_candles
+    db = SessionLocal()
+    try:
+        refresh_latest_candles(db)
+    finally:
+        db.close()
 
 
 @app.get("/")