fix: improve example image upload reliability and error handling, #804

- Sequential per-file upload to avoid client_max_size limits
- Add backend exception handler with proper 500 responses
- Increase standalone server upload limit to 256MB
- Add partial success localization support

py/routes/handlers/example_images_handlers.py

@@ -1,11 +1,14 @@
 """Handler set for example image routes."""
 from __future__ import annotations
 
+import logging
 from dataclasses import dataclass
 from typing import Callable, Mapping
 
 from aiohttp import web
 
+logger = logging.getLogger(__name__)
+
 from ...services.use_cases.example_images import (
     DownloadExampleImagesConfigurationError,
     DownloadExampleImagesInProgressError,
@@ -122,6 +125,9 @@ class ExampleImagesManagementHandler:
             return web.json_response({'success': False, 'error': str(exc)}, status=400)
         except ExampleImagesImportError as exc:
             return web.json_response({'success': False, 'error': str(exc)}, status=500)
+        except Exception as exc:
+            logger.exception("Unexpected error importing example images")
+            return web.json_response({'success': False, 'error': str(exc)}, status=500)
 
     async def delete_example_image(self, request: web.Request) -> web.StreamResponse:
         return await self._processor.delete_custom_image(request)