(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/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: