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/schemas.py

@@ -117,6 +117,7 @@ class SimulationResponse(BaseModel):
     status: str
     config: dict
     signals: Optional[List[dict]]
+    klines: Optional[List[dict]] = None  # Price data for chart
 
     class Config:
         from_attributes = True