(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/config_repository.py

@@ -85,7 +85,7 @@ def global_config_data(name: str = "default") -> dict[str, Any]:
     try:
         global_config = GlobalConfig.objects.get(name=name)
     except ObjectDoesNotExist as exc:
-        raise ConfigRepositoryError(f"Missing GlobalConfig {name!r}") from exc
+        raise ConfigRepositoryError(f"Missing global config {name!r}") from exc
     return _global_runtime_data(global_config)
 
 
@@ -93,5 +93,5 @@ def host_config_data(host: str) -> dict[str, Any]:
     try:
         host_config = HostConfig.objects.get(host=host, enabled=True)
     except ObjectDoesNotExist as exc:
-        raise ConfigRepositoryError(f"Missing enabled HostConfig {host!r}") from exc
+        raise ConfigRepositoryError(f"Missing enabled host {host!r}") from exc
     return _host_runtime_data(host_config)