fix(monitor): write Proxmox's notification settings only while /etc/pve is mounted

- Nothing is written under /etc/pve unless it is the mounted cluster
  filesystem; a Monitor that starts while pve-cluster is being restarted
  waits for it and registers its webhook afterwards.
- The Monitor service is ordered after pve-cluster.service.
- The settings are written only when they have to change. The copies
  taken before a change are kept in
  /usr/local/share/proxmenux/backups/pve-notifications, and the ones left
  in /etc/pve by earlier versions are moved there.
- Uninstalling the Monitor removes its target and matcher from Proxmox.

Reported in #427.

web/messages/en/docs/monitor/notifications.json

@@ -342,7 +342,7 @@
     "practiceBody": "The PVE setup writes the target as <code>http://127.0.0.1:8008</code>, so PVE-emitted notifications always go through the loopback path with rate-limit-only security. The remote-caller path with the shared secret is opt-in for custom integrations — point an external service at <code>https://&lt;monitor-host&gt;:&lt;port&gt;/api/notifications/webhook</code> and supply the <code>X-Webhook-Secret</code> header to use it.",
     "actionsIntro": "The Monitor manages this target through three actions on the Settings tab:",
     "actionsItems": [
-      "<strong>Setup</strong> — runs automatically when you enable Notifications. Creates the entry in <code>/etc/pve/notifications.cfg</code> after backing up the current file.",
+      "<strong>Setup</strong> — runs automatically when you enable Notifications. Creates the entry in <code>/etc/pve/notifications.cfg</code>. The file is written only when its content has to change, and a copy of the previous one is kept in <code>/usr/local/share/proxmenux/backups/pve-notifications/</code> (the three newest). Nothing is written while <code>/etc/pve</code> is not mounted: a Monitor that starts during a restart of <code>pve-cluster</code> registers the entry once it is back.",
       "<strong>Cleanup</strong> — removes the entry. The previous backup of the file is kept.",
       "<strong>Read config</strong> — shows the current targets and matchers as PVE sees them. This is how you confirm the Monitor's entry is the one firing when PVE has multiple notification routes configured."
     ],

web/messages/en/docs/settings/uninstall-proxmenux.json

@@ -19,7 +19,7 @@
     "items": [
       "<strong>Confirmation dialog.</strong> Yes/no — “Are you sure you want to uninstall ProxMenux?”",
       "<strong>Dependency removal checklist.</strong> Pre-selected list depends on install type (see table below). All boxes default to OFF — you opt in to removing each dependency.",
-      "<strong>Removal runs with a progress bar.</strong> Removes the venv (Translation install only), the <code>menu</code> launcher, the <code>/usr/local/share/proxmenux/</code> tree, the ProxMenux Monitor service if present, the dependencies you ticked, and finally restores the system file backups (see below).",
+      "<strong>Removal runs with a progress bar.</strong> Removes the venv (Translation install only), the <code>menu</code> launcher, the <code>/usr/local/share/proxmenux/</code> tree, the ProxMenux Monitor service if present together with the notification target it registered in Proxmox VE (<code>proxmenux-webhook</code> and its matcher), the dependencies you ticked, and finally restores the system file backups (see below).",
       "<strong>Completion dialog.</strong> Lists removed dependencies and exits."
     ]
   },