fix(validation): populate screenshot_paths column + backfill from raw_response

Two bugs preventing screenshot display in report UI:

1. Plugin's ValidationRecord constructor did NOT include
   screenshot_paths (plural, JSON array). The column stayed NULL
   even though paths were stored in raw_response. Records created
   by the plugin now include screenshot_paths=webp_paths|jpeg_paths.

2. Backward compat for existing records: _record_to_dict now
   falls back to extracting screenshot_paths from raw_response JSON
   when the DB column is empty.

Also fixes: broken import statement in validation_service.py
(missing closing paren + missing imports from earlier edit).

backend/src/plugins/llm_analysis/plugin.py

@@ -478,6 +478,7 @@ class DashboardValidationPlugin(PluginBase):
                 summary=validation_result.summary,
                 issues=[issue.model_dump() for issue in validation_result.issues],
                 screenshot_path=validation_result.screenshot_path,
+                screenshot_paths=webp_paths or jpeg_paths,
                 raw_response=json.dumps(result_payload, ensure_ascii=False),
             )
             db.add(db_record)