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/models.py

@@ -24,3 +24,14 @@ class Purchase(Base):
     user_id = Column(Integer, ForeignKey("users.id"), nullable=False)
 
     owner = relationship("User", back_populates="purchases")
+
+
+class OHLCCandle(Base):
+    __tablename__ = "ohlc_candles"
+
+    id    = Column(Integer, primary_key=True, index=True)
+    date  = Column(String, unique=True, index=True, nullable=False)  # "YYYY-MM-DD" UTC
+    open  = Column(Float, nullable=False)
+    high  = Column(Float, nullable=False)
+    low   = Column(Float, nullable=False)
+    close = Column(Float, nullable=False)