feat: add trade activity dashboard

Shows what happened at each candle:
- BUY/SELL/HOLD actions
- Price at that time
- Reason for action
- Entry price for positions

Trade log is stored in DB and displayed in frontend.

src/backend/app/db/schemas.py

@@ -118,6 +118,7 @@ class SimulationResponse(BaseModel):
     config: dict
     signals: Optional[List[dict]]
     klines: Optional[List[dict]] = None  # Price data for chart
+    trade_log: Optional[List[dict]] = None  # Trade activity log
 
     class Config:
         from_attributes = True