generate_thumbnails_with_progress is split into _thumbnail_targets
(scan, executor), _generate_one (one PIL/ffmpeg job, executor) and an
async driver that awaits each file via _run_in_executor, emits one
progress frame per file and yields between files. Per-file failures
are reported as file_error events (error stays fatal for the UI) and
the loop continues. The wildcard Access-Control-Allow-Origin header is
gone. The blocking POST variant reuses the same helpers. The scan never
follows symlinks and is capped by MAX_SCAN_DEPTH / MAX_THUMBNAIL_FILES.
Vito Sansevero
5fddf56eee