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/api/simulate.py

@@ -52,6 +52,8 @@ def run_simulation_sync(
                         {"time": k.get("time"), "close": k.get("close")}
                         for k in engine.klines
                     ]
+                    # Save trade log for dashboard
+                    simulation.trade_log = engine.trade_log
                     db.commit()
 
                 for signal in engine.signals: