(refactor) Remove pobsync_home from global config

Drop the obsolete pobsync_home field from GlobalConfig and remove it from
runtime config generation, form saves, and configuration commands.

The runtime state root now comes exclusively from POBSYNC_HOME/settings,
which keeps the Django model focused on backup behavior instead of install
layout.

src/pobsync_backend/config_repository.py

@@ -17,7 +17,6 @@ class ConfigRepositoryError(RuntimeError):
 def _global_runtime_data(global_config: GlobalConfig) -> dict[str, Any]:
     data = {
         "backup_root": global_config.backup_root,
-        "pobsync_home": global_config.pobsync_home,
         "ssh": {
             "user": global_config.ssh_user,
             "port": global_config.ssh_port,