(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/tests/test_views.py

@@ -475,7 +475,6 @@ 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.default_ssh_credential, credential)
         self.assertEqual(config.ssh_user, "backup")
         self.assertEqual(config.ssh_port, 2222)
@@ -502,7 +501,6 @@ class ViewTests(TestCase):
         GlobalConfig.objects.create(
             name="default",
             backup_root="/mnt/pobsync/backups",
-            pobsync_home="/custom/state/home",
         )
 
         response = self.client.get(reverse("edit_global_config"))
@@ -532,7 +530,6 @@ class ViewTests(TestCase):
         GlobalConfig.objects.create(
             name="default",
             backup_root="/mnt/pobsync/backups",
-            pobsync_home="/custom/state/home",
         )
 
         response = self.client.post(
@@ -561,7 +558,6 @@ class ViewTests(TestCase):
         self.assertRedirects(response, reverse("dashboard"))
         config = GlobalConfig.objects.get(name="default")
         self.assertEqual(config.backup_root, "/backups")
-        self.assertEqual(config.pobsync_home, "/opt/pobsync")
 
     def test_create_host_config_form_creates_host(self) -> None:
         self.client.force_login(self.staff_user)