(refactor) Use operator-facing config errors

Replace remaining model-name based configuration errors with labels that
match the Django-first operating model.

Add coverage for missing global config and host configuration errors so
operator-facing messages stay readable.

src/pobsync_backend/tests/test_config_repository.py

@@ -2,7 +2,7 @@ from __future__ import annotations
 
 from django.test import TestCase
 
-from pobsync_backend.config_repository import global_config_data, host_config_data
+from pobsync_backend.config_repository import ConfigRepositoryError, global_config_data, host_config_data
 from pobsync_backend.models import GlobalConfig, HostConfig
 
 
@@ -52,3 +52,12 @@ class ConfigRepositoryTests(TestCase):
         self.assertEqual(host_cfg["excludes_add"], ["/tmp/***"])
         self.assertNotIn("unknown", global_cfg)
         self.assertNotIn("unknown", host_cfg)
+
+    def test_missing_config_errors_use_operator_labels(self) -> None:
+        with self.assertRaisesMessage(ConfigRepositoryError, "Missing global config 'default'"):
+            global_config_data()
+
+        GlobalConfig.objects.create(name="default", backup_root="/backups")
+
+        with self.assertRaisesMessage(ConfigRepositoryError, "Missing enabled host 'web-01'"):
+            host_config_data("web-01")