(bugfix) preserve saved global backup root in Django setup form

Fix the global config edit view so default initial values are only used
when creating a new config, preventing saved backup_root values from
being hidden by form defaults.

Keep pobsync_home as an internal runtime setting instead of exposing it
in the normal Django setup form.

Mount a host backup directory into Docker at /backups and document
POBSYNC_BACKUP_ROOT so backup_root behaves predictably in containers.

src/pobsync_backend/views.py

@@ -51,7 +51,7 @@ def edit_global_config(request):
             messages.success(request, f"Global config saved for {saved_config.name}.")
             return redirect("dashboard")
     else:
-        form = GlobalConfigForm(instance=global_config, initial=_default_global_initial())
+        form = GlobalConfigForm(instance=global_config) if global_config else GlobalConfigForm(initial=_default_global_initial())
 
     return render(
         request,
@@ -235,8 +235,7 @@ def _default_schedule_initial() -> dict[str, object]:
 def _default_global_initial() -> dict[str, object]:
     return {
         "name": "default",
-        "backup_root": "/opt/pobsync/backups",
-        "pobsync_home": "/opt/pobsync",
+        "backup_root": "/backups",
         "ssh_user": "root",
         "ssh_port": 22,
         "rsync_binary": "rsync",