fix(api): treat transient server errors (524/5xx) as non-fatal in model updates (#935)

Teach CivitaiClient.get_model_versions() to recognise Cloudflare 524, generic
5xx, and connection-level errors as transient failures and return None
instead of raising RuntimeError, so a single upstream glitch does not
block the entire batch update or produce a scary traceback.

Also downgrade the generic except Exception log level in
ModelUpdateService._refresh_single_model() from error (with exc_info)
to warning (message only), since the full traceback is already logged
upstream in CivitaiClient.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

py/services/model_update_service.py

@@ -1000,12 +1000,11 @@ class ModelUpdateService:
                     fallback_error_message = str(exc) or "resource not found"
                     mark_model_as_ignored = True
                 except Exception as exc:  # pragma: no cover - defensive log
-                    logger.error(
+                    logger.warning(
                         "Failed to fetch versions for model %s (%s): %s",
                         model_id,
                         model_type,
                         exc,
-                        exc_info=True,
                     )
                     fallback_error_message = str(exc)
             if response is not None: