feat: add price chart to simulation and unit tests

Unit tests (13 passing):
- Kline fetching and processing
- Price drop condition triggers buy
- Stop loss and take profit risk management
- Multiple positions (buy again after sell)
- Max candles limit
- Stop interruption handling

Frontend:
- SignalChart now shows price movement even before signals
- Shows candle count even with no signals
- Chart displays buy/sell markers when signals exist
- Canvas-based chart with gradient fill

Backend:
- Simulation stores klines for chart display
- Returns klines in API response
- Simplified simulation run (no periodic saving)

src/backend/app/db/models.py

@@ -93,6 +93,7 @@ class Simulation(Base):
     status = Column(String, nullable=False)
     config = Column(JSON, nullable=False)
     signals = Column(JSON)
+    klines = Column(JSON)  # Price data for chart display
 
     bot = relationship("Bot", back_populates="simulations")