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
- Add optional `media_type_hint` parameter to `_get_file_extension_from_content_or_headers` method
- When `media_type_hint` is "video" and no extension can be determined from content/headers/URL, default to `.mp4`
- Pass image metadata type as hint in both `process_example_images` and `process_example_images_batch` methods
- Add unit tests to verify media type hint behavior and priority
