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_test.go

@@ -175,10 +175,13 @@ func TestChartYAxisNumber(t *testing.T) {
 	}{
 		{in: 999, want: "999"},
 		{in: 1000, want: "1к"},
-		{in: 1370, want: "1к"},
-		{in: 1500, want: "2к"},
+		{in: 1370, want: "1,4к"},
+		{in: 1500, want: "1,5к"},
+		{in: 1700, want: "1,7к"},
+		{in: 2000, want: "2к"},
+		{in: 9999, want: "10к"},
 		{in: 10200, want: "10к"},
-		{in: -1499, want: "-1к"},
+		{in: -1500, want: "-1,5к"},
 	}
 	for _, tc := range tests {
 		if got := chartYAxisNumber(tc.in); got != tc.want {