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/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)