feat(varaverk): add local .plg registration; remove dev-mode cron fallback

Create varaverk.plg on flash (/boot/config/plugins/varaverk/varaverk.plg).
On boot, rebuild_cron copies it to /var/log/plugins/ so update_cron natively
picks up varaverk.cron and merges it into /etc/cron.d/root.

With the .plg in place the dev-mode direct-inject fallback in vv_cron_rebuild()
is no longer needed and has been removed.

Plugin/usr/local/emhttp/plugins/varaverk/event/disks_mounted/rebuild_cron

@@ -1,6 +1,7 @@
 #!/bin/bash
-# Rebuild /etc/cron.d/varaverk from schedule.json after every boot.
-# /etc/cron.d/ is RAM-based and wiped on reboot; schedule.json persists on flash.
+# Register plugin with Unraid's cron system and rebuild cron entries after every boot.
+# /var/log/plugins/ and /etc/cron.d/ are RAM-based and wiped on reboot.
+cp /boot/config/plugins/varaverk/varaverk.plg /var/log/plugins/varaverk.plg 2>/dev/null
 php -r "
 require_once '/usr/local/emhttp/plugins/varaverk/include/scheduler.php';
 \$schedule = vv_schedule_load();