feat(pbs): post-install menu for Proxmox Backup Server

- Open the post-install menu from the PBS menu
- List only the customizable options that apply to PBS
- Offer the PBS no-subscription repository through the native repository API
- Upgrade PBS with the package list shown before applying it
- Remove and restore the subscription notice on the PBS web interface
- Install Figurine on PBS without the Proxmox VE aliases
- Offer the Helper-Scripts PBS post-install and microcode scripts
- Tune firewall bridges after the systemd udev rules in the automated post-install
- Add PBS repository policy tests and syntax checks
- Document the PBS differences on the post-install page
- Use backups in the Spanish PBS update message

AppImage/scripts/flask_notification_routes.py

@@ -1156,6 +1156,14 @@ def setup_pve_webhook_core() -> dict:
         'fallback_commands': [],
         'error': None,
     }
+    try:  # Proxmox Backup Server has no /etc/pve notification configuration
+        from pbs_manager import is_pbs_host
+        on_pbs = is_pbs_host()
+    except Exception:
+        on_pbs = False
+    if on_pbs:
+        result['not_applicable'] = True
+        return result
     
     try:
         if not _pve_cluster_fs_mounted():
@@ -1339,6 +1347,13 @@ def cleanup_pve_webhook_core() -> dict:
     Only removes blocks named 'proxmenux-webhook' / 'proxmenux-default'.
     """
     result = {'cleaned': False, 'error': None}
+    try:  # Proxmox Backup Server has no /etc/pve notification configuration
+        from pbs_manager import is_pbs_host
+        on_pbs = is_pbs_host()
+    except Exception:
+        on_pbs = False
+    if on_pbs:
+        return {'cleaned': True, 'error': None, 'not_applicable': True}
     
     try:
         if not _pve_cluster_fs_mounted():