(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.

.gitignore

@@ -2,6 +2,7 @@ __pycache__/
 *.py[cod]
 .venv/
 var/
+backups/
 .pytest_cache/
 .mypy_cache/
 *.egg-info/