#6 Add multi-timeframe support for candles

Implement support for multiple timeframes (15m, 30m, 1h, 1D, 1W, 1M) in candle data. Rename Candle1h model to Candle and add timeframe field to database schema. Update API routes, controllers, and tests to handle timeframe parameter. Modify MT5 expert to send historical and live data for all supported timeframes. Add .env.example with configuration templates.