fix: P1-3 address heartbeat timer leak, test drain, and store consistency

- COR-001: drain runHeartbeat goroutine via done channel before test returns,
  eliminating race against concurrent SetHeartbeatInterval tests
- COR-002: replace time.After with time.NewTimer + defer t.Stop() + t.Reset()
  to prevent timer goroutine leaks on context cancellation
- COR-005: guard HeartbeatInterval store write with > 0 check in OnClusterUpdated,
  keeping store consistent with SetHeartbeatInterval's zero-ignore semantics

src/frontend/apiserver/backend/handler.go

@@ -60,7 +60,9 @@ func (h *DefaultHandler) OnHostConfigUpdated(incoming *models.HostInfo) {
 func (h *DefaultHandler) OnClusterUpdated(incoming *models.ClusterInfo) {
 	existing, _ := store.Clusters.Get(incoming.ClusterName)
 	existing.ClusterName = incoming.ClusterName
-	existing.HeartbeatInterval = incoming.HeartbeatInterval
+	if incoming.HeartbeatInterval > 0 {
+		existing.HeartbeatInterval = incoming.HeartbeatInterval
+	}
 	existing.Vcpus = incoming.Vcpus
 	existing.InactiveHeartbeatCount = incoming.InactiveHeartbeatCount
 	store.Clusters.Put(existing.ClusterName, existing)

src/frontend/apiserver/backend/heartbeat.go

@@ -21,13 +21,17 @@ func SetHeartbeatInterval(secs int) {
 // runHeartbeat loops until ctx is cancelled, calling fn(intervalSecs) on each tick.
 // Extracted from clib.go for testability without CGO.
 func runHeartbeat(ctx context.Context, fn func(int)) {
+	interval := int(atomic.LoadInt32(&heartbeatIntervalSecs))
+	t := time.NewTimer(time.Duration(interval) * time.Second)
+	defer t.Stop()
 	for {
-		interval := int(atomic.LoadInt32(&heartbeatIntervalSecs))
 		select {
 		case <-ctx.Done():
 			return
-		case <-time.After(time.Duration(interval) * time.Second):
+		case <-t.C:
 			fn(interval)
 		}
+		interval = int(atomic.LoadInt32(&heartbeatIntervalSecs))
+		t.Reset(time.Duration(interval) * time.Second)
 	}
 }

src/frontend/apiserver/backend/heartbeat_test.go

@@ -35,17 +35,18 @@ func TestRunHeartbeat_CallsFunction(t *testing.T) {
 	atomic.StoreInt32(&heartbeatIntervalSecs, 1)
 
 	var callCount int64
-	fn := func(_ int) {
-		atomic.AddInt64(&callCount, 1)
-	}
+	fn := func(_ int) { atomic.AddInt64(&callCount, 1) }
 
 	ctx, cancel := context.WithCancel(context.Background())
-	defer cancel()
-
-	go runHeartbeat(ctx, fn)
+	done := make(chan struct{})
+	go func() {
+		runHeartbeat(ctx, fn)
+		close(done)
+	}()
 
 	time.Sleep(1500 * time.Millisecond)
 	cancel()
+	<-done // wait for goroutine to exit before the test returns
 
 	if got := atomic.LoadInt64(&callCount); got < 1 {
 		t.Errorf("fn was called %d times, want at least 1", got)