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/platform/kill_workers.go

@@ -0,0 +1,64 @@
+package platform
+
+import (
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+	"syscall"
+)
+
+// workerPatterns are substrings matched against /proc/<pid>/cmdline to identify
+// bee test worker processes that should be killed by KillTestWorkers.
+var workerPatterns = []string{
+	"bee-gpu-burn",
+	"stress-ng",
+	"stressapptest",
+	"memtester",
+}
+
+// KilledProcess describes a process that was sent SIGKILL.
+type KilledProcess struct {
+	PID  int    `json:"pid"`
+	Name string `json:"name"`
+}
+
+// KillTestWorkers scans /proc for running test worker processes and sends
+// SIGKILL to each one found. It returns a list of killed processes.
+// Errors for individual processes (e.g. already exited) are silently ignored.
+func KillTestWorkers() []KilledProcess {
+	entries, err := os.ReadDir("/proc")
+	if err != nil {
+		return nil
+	}
+
+	var killed []KilledProcess
+	for _, e := range entries {
+		if !e.IsDir() {
+			continue
+		}
+		pid, err := strconv.Atoi(e.Name())
+		if err != nil {
+			continue
+		}
+		cmdline, err := os.ReadFile(fmt.Sprintf("/proc/%d/cmdline", pid))
+		if err != nil {
+			continue
+		}
+		// /proc/*/cmdline uses NUL bytes as argument separators.
+		args := strings.SplitN(strings.ReplaceAll(string(cmdline), "\x00", " "), " ", 2)
+		exe := strings.TrimSpace(args[0])
+		base := exe
+		if idx := strings.LastIndexByte(exe, '/'); idx >= 0 {
+			base = exe[idx+1:]
+		}
+		for _, pat := range workerPatterns {
+			if strings.Contains(base, pat) || strings.Contains(exe, pat) {
+				_ = syscall.Kill(pid, syscall.SIGKILL)
+				killed = append(killed, KilledProcess{PID: pid, Name: base})
+				break
+			}
+		}
+	}
+	return killed
+}