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

main.go

@@ -50,6 +50,9 @@ func main() {
 	// Initialize dashboard (web server + SSE)
 	dashboard := NewDashboard(store, trader, database, ":8888")
 	go dashboard.Run()
+
+	// P3-4: wire real-time trade event broadcast
+	trader.OnTradeEvent = dashboard.BroadcastEvent
 	if trader.IsConfigured() {
 		modeLabel := trader.ModeLabel()
 		log.Printf("[Trader] %s mode: automated trading ENABLED (threshold >= %.2f%%, $%.0f/trade)",
@@ -85,6 +88,7 @@ func main() {
 				err := runner(func(coin string, price, bid, ask float64) {
 					store.SetWithSpread(coin, name, price, bid, ask)
 					dashboard.RecordPrice(coin, name, price)
+					dashboard.RecordConnStatus(name) // P3-5
 				})
 				log.Printf("[%s] WS error: %v (reconnecting...)", name, err)
 				select {