Phase 3: Real-time enhancements

P3-1: Scan optimization — only BG↔HL (50+ pair combos → 2)
P3-2: Real-time spread chart — spreadHistory ring buffer +
      /api/spread-history endpoint + Chart.js spread chart
P3-3: Live position PnL — positions SSE now includes
      estimated current profit/loss + current spread
P3-4: Real-time trade events — trader.OnTradeEvent callback
      fires SSE 'trade_open' / 'trade_close' immediately
P3-5: Connection status monitoring — tracks last update time
      per exchange, broadcast via stats.connections + /api/connections

Frontend: spread chart card, PnL column in positions,
          connection status dots in stats bar,
          green/red border flash on trade events

trader.go

@@ -63,6 +63,8 @@ type Trader struct {
 	positions     map[string]*ArbPosition // coin -> position
 	lastTradeTime map[string]time.Time
 	closedTrades  []TradeRecord // history of closed trades
+
+	OnTradeEvent func(event string, data interface{}) // P3-4: real-time SSE push
 }
 
 // TradeRecord stores a finalized trade for stats tracking.
@@ -277,6 +279,17 @@ func (t *Trader) executeEntry(opp *ArbOpportunity, store *PriceStore, notifier *
 		pos.LongLeg.Exchange, pos.LongLeg.EntryPrice,
 		pos.ShortLeg.Exchange, pos.ShortLeg.EntryPrice,
 		diff, t.cfg.TradeAmountUSD))
+
+	// P3-4: real-time trade event push
+	if t.OnTradeEvent != nil {
+		t.OnTradeEvent("trade_open", map[string]interface{}{
+			"coin":         pos.Coin,
+			"direction":    pos.Direction,
+			"entry_spread": diff,
+			"amount_usd":   t.cfg.TradeAmountUSD,
+			"time":         time.Now().Format("15:04:05"),
+		})
+	}
 }
 
 // checkScaleIn adds more position when spread widens further.
@@ -432,6 +445,20 @@ func (t *Trader) checkExit(pos *ArbPosition, bgP, hlP, diffPct float64, notifier
 		msg += fmt.Sprintf("  平仓异常: %s\n", closeErr)
 	}
 	notifier.Send(msg)
+
+	// P3-4: real-time trade event push
+	if t.OnTradeEvent != nil {
+		t.OnTradeEvent("trade_close", map[string]interface{}{
+			"coin":          pos.Coin,
+			"direction":     pos.Direction,
+			"entry_spread":  pos.EntrySpread,
+			"exit_spread":   diffPct,
+			"pnl_pct":       netPnl,
+			"convergence":   convergenceLabel,
+			"duration":      elapsed.Round(time.Second).String(),
+			"time":          time.Now().Format("15:04:05"),
+		})
+	}
 }
 
 func (t *Trader) placeOrder(leg *PositionLeg, side string, store *PriceStore) string {