Cache: warm at array start, extend stale tolerance to 5 min, fix last uptime copy

- ARRAY_START_SCRIPTS: add api_cache_writer.sh so monitor/arrs cache is populated
  immediately on array start (/tmp is tmpfs — cleared on reboot, so first-boot
  load was hitting live API for up to 60 seconds until the cron fired)
- api/monitor.php + arrs.php: raise cache TTL from 90s to 300s — stale-but-instant
  beats a 6-second live API wait if the writer is momentarily behind
- include/unraid_api.php: replace last inline uptime formatter with vv_format_uptime()

Plugin/unraid/include/unraid_api.php

@@ -216,9 +216,8 @@ function vv_local_host_stats(): array {
 
     $uptimeRaw = $os['uptime'] ?? '';
     if (is_numeric($uptimeRaw)) {
-        $s = (int)$uptimeRaw;
-        $d = intdiv($s, 86400); $h = intdiv($s % 86400, 3600); $m = intdiv($s % 3600, 60);
-        $uptime = ($d ? "{$d}d " : '') . ($h ? "{$h}h " : '') . "{$m}m";
+        $s      = (int)$uptimeRaw;
+        $uptime = vv_format_uptime($s);
     } else {
         $s = 0; $uptime = $uptimeRaw ?: '—';
     }