(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_configure_commands.py

@@ -16,7 +16,6 @@ class ConfigureCommandsTests(TestCase):
         call_command(
             "configure_pobsync_global",
             backup_root="/backups",
-            pobsync_home="/opt/pobsync",
             retention="daily=3,weekly=2,monthly=1,yearly=0",
             stdout=out,
         )