scheduler: add Array Starting / Array Stopping event triggers

Two hardcoded event-triggered entries appear at the top of the scheduler
(array_started.sh / array_stopping.sh). They show an ⚡ Array Start /
⚡ Array Stop badge instead of a cron field and are enabled/disabled via
the normal toggle.

Static event scripts fire them via Unraid's event system:
- event/disks_mounted/array_start_jobs  → runs in background (non-blocking)
- event/disks_unmounting/array_stop_jobs → runs foreground (blocks until done)

vv_cron_rebuild() skips @array_* entries so they never land in the cron file.
Scripts executed on each event are managed in master.conf via
ARRAY_START_SCRIPTS and ARRAY_STOP_SCRIPTS arrays.

Plugin/usr/local/emhttp/plugins/varaverk/api/scheduler.php

@@ -12,8 +12,9 @@ if (!$id) {
     exit;
 }
 
-// Basic cron validation — 5 fields or empty
-if ($cron && !preg_match('/^(\S+\s+){4}\S+$/', $cron)) {
+// Basic cron validation — 5 fields, or known @event trigger, or empty
+if ($cron && !in_array($cron, ['@array_start', '@array_stop'], true)
+         && !preg_match('/^(\S+\s+){4}\S+$/', $cron)) {
     echo json_encode(['ok' => false, 'error' => 'Invalid cron expression']);
     exit;
 }