fix(trader): stop order-sync goroutine leak and rate-limit hammering

Every StartOrderSync spawned a ticker goroutine that ran forever — it
survived trader stop AND deletion, so each quick-created trader left a
permanent 30s Hyperliquid poll behind. Stacked leaks turned into an
~8s effective hammer that tripped Hyperliquid's 429 rate limit, which
then broke the symbol board, trader creation, and order sync itself.

- new trader/syncloop package: shared stoppable sync loop with
  exponential failure backoff (30s base, 5min cap)
- all 9 exchanges' StartOrderSync now take the trader's stop channel
  and stop when the trader stops (close broadcast from AutoTrader.Stop)
- provider/hyperliquid: GetPerpDexCoins now serves a 5min TTL cache and
  falls back to the stale board when the upstream returns 429, so the
  symbol panel keeps working through rate limiting

trader/syncloop/syncloop.go

@@ -0,0 +1,48 @@
+// Package syncloop runs background exchange order-sync loops with a shared
+// lifecycle: loops stop when the owning trader stops, and consecutive
+// failures back off exponentially so a rate-limiting exchange (HTTP 429)
+// is not hammered at full frequency.
+package syncloop
+
+import (
+	"time"
+
+	"nofx/logger"
+)
+
+// maxBackoff caps the failure backoff so a recovered exchange is picked up
+// within a few minutes at worst.
+const maxBackoff = 5 * time.Minute
+
+// Run starts a background loop calling syncFn at the given interval until
+// stop is closed. After each consecutive failure the wait doubles (capped at
+// maxBackoff); the first success resets it to the base interval.
+func Run(stop <-chan struct{}, interval time.Duration, name string, syncFn func() error) {
+	if interval <= 0 {
+		interval = 30 * time.Second
+	}
+	go func() {
+		wait := interval
+		timer := time.NewTimer(wait)
+		defer timer.Stop()
+		for {
+			select {
+			case <-stop:
+				logger.Infof("⏹ %s order sync stopped", name)
+				return
+			case <-timer.C:
+				if err := syncFn(); err != nil {
+					wait *= 2
+					if wait > maxBackoff {
+						wait = maxBackoff
+					}
+					logger.Infof("⚠️ %s order sync failed: %v (backing off, next attempt in %v)", name, err, wait)
+				} else {
+					wait = interval
+				}
+				timer.Reset(wait)
+			}
+		}
+	}()
+	logger.Infof("🔄 %s order sync started (interval: %v)", name, interval)
+}