(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_django_config_source.py

@@ -15,7 +15,6 @@ class DjangoConfigSourceTests(TestCase):
         GlobalConfig.objects.create(
             name="default",
             backup_root="/backups",
-            pobsync_home="/opt/pobsync",
             rsync_args=["--archive"],
             rsync_extra_args=["--numeric-ids"],
             excludes_default=["/proc/***"],
@@ -25,7 +24,6 @@ class DjangoConfigSourceTests(TestCase):
             retention_yearly=1,
             data={
                 "backup_root": "/ignored",
-                "pobsync_home": "/ignored",
                 "ssh": {"user": "root", "port": 22, "options": []},
                 "rsync": {
                     "binary": "rsync",
@@ -72,7 +70,6 @@ class DjangoConfigSourceTests(TestCase):
         GlobalConfig.objects.create(
             name="default",
             backup_root="/backups",
-            pobsync_home="/opt/pobsync",
             default_ssh_credential=credential,
             ssh_options=["-oBatchMode=yes"],
         )
@@ -99,7 +96,6 @@ class DjangoConfigSourceTests(TestCase):
         GlobalConfig.objects.create(
             name="default",
             backup_root="/backups",
-            pobsync_home="/opt/pobsync",
             default_ssh_credential=global_credential,
         )
         HostConfig.objects.create(
@@ -127,7 +123,6 @@ class DjangoConfigSourceTests(TestCase):
             GlobalConfig.objects.create(
                 name="default",
                 backup_root="/backups",
-                pobsync_home="/opt/pobsync",
                 default_ssh_credential=credential,
             )
             HostConfig.objects.create(host="web-01", address="web-01.example.test")
@@ -146,7 +141,6 @@ class DjangoConfigSourceTests(TestCase):
             GlobalConfig.objects.create(
                 name="default",
                 backup_root="/backups",
-                pobsync_home="/opt/pobsync",
                 default_ssh_credential=credential,
             )
             HostConfig.objects.create(host="web-01", address="web-01.example.test")