(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/tests/test_admin.py

@@ -10,7 +10,7 @@ from pobsync_backend.models import BackupRun, GlobalConfig, HostConfig, Schedule
 
 
 class AdminDisplayTests(TestCase):
-    def test_admin_hides_old_global_state_field_and_uses_compatibility_label(self) -> None:
+    def test_admin_hides_old_global_state_fields_and_labels_host_runtime_state(self) -> None:
         site = AdminSite()
         global_admin = GlobalConfigAdmin(GlobalConfig, site)
         host_admin = HostConfigAdmin(HostConfig, site)
@@ -21,7 +21,9 @@ class AdminDisplayTests(TestCase):
         fieldset_names = [name for name, _options in [*global_fieldsets, *host_fieldsets]]
 
         self.assertNotIn("pobsync_home", global_fields)
-        self.assertIn("Compatibility data", fieldset_names)
+        self.assertNotIn("data", global_fields)
+        self.assertIn("Runtime state", fieldset_names)
+        self.assertNotIn("Compatibility data", fieldset_names)
         self.assertNotIn("Legacy JSON", fieldset_names)
 
     def test_host_admin_links_to_related_snapshots_and_runs(self) -> None: