(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/migrations/0010_remove_globalconfig_pobsync_home.py

@@ -0,0 +1,14 @@
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("pobsync_backend", "0009_remove_scheduleconfig_user"),
+    ]
+
+    operations = [
+        migrations.RemoveField(
+            model_name="globalconfig",
+            name="pobsync_home",
+        ),
+    ]