Split the single-model and bulk context menu actions into 'Download
Missing Example Images' (regular endpoint, skips already-processed
models) and 'Re-process Example Images' (force endpoint, retries
failed models).
- start_download accepts model_hashes so a selected subset can be
processed with the progress-aware skip logic; explicitly targeted
models bypass the failed/processed model-level guards so per-image
gaps are filled
- pre-download existence check in the processor skips network requests
for image files already on disk across all download paths
- force download retries previously failed models and clears their
failed status on success
- add i18n keys for the new menu items across all locales
9087b4b07c