(refactor) Remove obsolete global config JSON storage

Drop the unused GlobalConfig.data field and remove the remaining YAML
config path helpers from PobsyncPaths.

Keep HostConfig.config as runtime state for preflight data, and relabel it
in the admin so it no longer reads as legacy compatibility storage.

src/pobsync_backend/migrations/0011_remove_globalconfig_data.py

@@ -0,0 +1,14 @@
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("pobsync_backend", "0010_remove_globalconfig_pobsync_home"),
+    ]
+
+    operations = [
+        migrations.RemoveField(
+            model_name="globalconfig",
+            name="data",
+        ),
+    ]