(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/forms.py

@@ -119,7 +119,6 @@ class GlobalConfigForm(forms.ModelForm):
     def save(self, commit: bool = True):
         instance = super().save(commit=False)
         instance.backup_root = settings.POBSYNC_BACKUP_ROOT
-        instance.pobsync_home = settings.POBSYNC_HOME
         if commit:
             instance.save()
             self.save_m2m()