(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(

src/pobsync_backend/tests/test_views.py

@@ -76,7 +76,6 @@ class ViewTests(TestCase):
             {
                 "name": "default",
                 "backup_root": "/backups",
-                "pobsync_home": "/opt/pobsync",
                 "ssh_user": "backup",
                 "ssh_port": "2222",
                 "ssh_options": "StrictHostKeyChecking=no\nBatchMode=yes",
@@ -100,6 +99,7 @@ class ViewTests(TestCase):
         self.assertContains(response, "Global config saved for default.")
         config = GlobalConfig.objects.get(name="default")
         self.assertEqual(config.backup_root, "/backups")
+        self.assertEqual(config.pobsync_home, "/opt/pobsync")
         self.assertEqual(config.ssh_user, "backup")
         self.assertEqual(config.ssh_port, 2222)
         self.assertEqual(config.ssh_options, ["StrictHostKeyChecking=no", "BatchMode=yes"])
@@ -109,6 +109,21 @@ class ViewTests(TestCase):
         self.assertEqual(config.retention_daily, 7)
         self.assertEqual(config.retention_yearly, 1)
 
+    def test_global_config_form_renders_saved_backup_root_on_edit(self) -> None:
+        self.client.force_login(self.staff_user)
+        GlobalConfig.objects.create(
+            name="default",
+            backup_root="/mnt/pobsync/backups",
+            pobsync_home="/custom/legacy/home",
+        )
+
+        response = self.client.get(reverse("edit_global_config"))
+
+        self.assertEqual(response.status_code, 200)
+        self.assertContains(response, "/mnt/pobsync/backups")
+        self.assertNotContains(response, "/opt/pobsync/backups")
+        self.assertNotContains(response, "Pobsync home")
+
     def test_create_host_config_form_creates_host(self) -> None:
         self.client.force_login(self.staff_user)
 
@@ -196,7 +211,7 @@ class ViewTests(TestCase):
         self.assertContains(response, "web-01")
         self.assertContains(response, "success")
         self.assertContains(response, "ABCDEFGH")
-        self.assertContains(response, '"ok": true')
+        self.assertContains(response, ""ok": true")
         self.assertContains(response, reverse("snapshot_detail", args=[snapshot.id]))
 
     def test_snapshot_detail_renders_metadata_runs_and_children(self) -> None:

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",