Add UPS email notifications and automatic UPS shutdown

- Add email notifications via PVE::Notify for all UPS events:
  - ONBATT: when UPS switches to battery
  - ONLINE: when power is restored
  - LOWBATT: critical battery level
  - SHUTDOWN_START/NODE/PRIMARY: during cluster shutdown
  - COMMBAD: communication lost with UPS

- Add automatic UPS shutdown command after cluster shutdown
  (protects against power surge when power returns)

- Update upssched.conf with ONLINE handler and immediate ONBATT notification

- Add notification templates for HTML and text emails

- Update documentation with new features and timer configuration

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

proxmox/ups/config/notification-templates/ups-power-event-subject.txt.hbs

@@ -0,0 +1 @@
+[{{ hostname }}] UPS {{ event_type }} - {{ event_title }}