(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/forms.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 from django import forms
+from django.conf import settings
 
 from .models import GlobalConfig, HostConfig, ScheduleConfig
 from .scheduler import parse_cron_expr
@@ -84,7 +85,6 @@ class GlobalConfigForm(forms.ModelForm):
         fields = (
             "name",
             "backup_root",
-            "pobsync_home",
             "ssh_user",
             "ssh_port",
             "ssh_options",
@@ -104,11 +104,18 @@ class GlobalConfigForm(forms.ModelForm):
         help_texts = {
             "name": "Usually 'default'. The backup engine currently reads the default config.",
             "backup_root": "Directory that contains host backup folders.",
-            "pobsync_home": "Base directory for runtime state inside the container or host.",
             "default_source_root": "Used by hosts without a custom source root.",
             "default_destination_subdir": "Optional subdirectory below each snapshot.",
         }
 
+    def save(self, commit: bool = True):
+        instance = super().save(commit=False)
+        instance.pobsync_home = settings.POBSYNC_HOME
+        if commit:
+            instance.save()
+            self.save_m2m()
+        return instance
+
 
 class ScheduleConfigForm(forms.ModelForm):
     cron_expr = forms.CharField(