fix(webui): prevent orphaned workers on restart, reduce metrics polling, add Kill Workers button

- tasks: mark TaskRunning tasks as TaskFailed on bee-web restart instead of
  re-queueing them — prevents duplicate gpu-burn-worker spawns when bee-web
  crashes mid-test (each restart was launching a new set of 8 workers on top
  of still-alive orphans from the previous crash)
- server: reduce metrics collector interval 1s→5s, grow ring buffer to 360
  samples (30 min); cuts nvidia-smi/ipmitool/sensors subprocess rate by 5×
- platform: add KillTestWorkers() — scans /proc and SIGKILLs bee-gpu-burn,
  stress-ng, stressapptest, memtester without relying on pkill/killall
- webui: add "Kill Workers" button next to Cancel All; calls
  POST /api/tasks/kill-workers which cancels the task queue then kills
  orphaned OS-level processes; shows toast with killed count
- metricsdb: sort GPU indices and fan/temp names after map iteration to fix
  non-deterministic sample reconstruction order (flaky test)
- server: fix chartYAxisNumber to use one decimal place for 1000–9999
  (e.g. "1,7к" instead of "2к") so Y-axis ticks are distinguishable

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

audit/internal/webui/server.go

@@ -128,7 +128,11 @@ type namedMetricsRing struct {
 	Ring *metricsRing
 }
 
-const metricsChartWindow = 120
+// metricsChartWindow is the number of samples kept in the live ring buffer.
+// At metricsCollectInterval = 5 s this covers 30 minutes of live history.
+const metricsChartWindow = 360
+
+var metricsCollectInterval = 5 * time.Second
 
 // pendingNetChange tracks a network state change awaiting confirmation.
 type pendingNetChange struct {
@@ -238,6 +242,7 @@ func NewHandler(opts HandlerOptions) http.Handler {
 	// Tasks
 	mux.HandleFunc("GET /api/tasks", h.handleAPITasksList)
 	mux.HandleFunc("POST /api/tasks/cancel-all", h.handleAPITasksCancelAll)
+	mux.HandleFunc("POST /api/tasks/kill-workers", h.handleAPITasksKillWorkers)
 	mux.HandleFunc("POST /api/tasks/{id}/cancel", h.handleAPITasksCancel)
 	mux.HandleFunc("POST /api/tasks/{id}/priority", h.handleAPITasksPriority)
 	mux.HandleFunc("GET /api/tasks/{id}/stream", h.handleAPITasksStream)
@@ -301,7 +306,7 @@ func NewHandler(opts HandlerOptions) http.Handler {
 
 func (h *handler) startMetricsCollector() {
 	go func() {
-		ticker := time.NewTicker(1 * time.Second)
+		ticker := time.NewTicker(metricsCollectInterval)
 		defer ticker.Stop()
 		for range ticker.C {
 			sample := platform.SampleLiveMetrics()
@@ -1059,9 +1064,16 @@ func chartYAxisNumber(v float64) string {
 		v = -v
 	}
 	var out string
-	if v >= 1000 {
+	switch {
+	case v >= 10000:
 		out = fmt.Sprintf("%dк", int((v+500)/1000))
-	} else {
+	case v >= 1000:
+		// Use one decimal place so ticks like 1400, 1600, 1800 read as
+		// "1,4к", "1,6к", "1,8к" instead of the ambiguous "1к"/"2к".
+		s := fmt.Sprintf("%.1f", v/1000)
+		s = strings.TrimRight(strings.TrimRight(s, "0"), ".")
+		out = strings.ReplaceAll(s, ".", ",") + "к"
+	default:
 		out = fmt.Sprintf("%.0f", v)
 	}
 	if neg {