The save_file_safely function may add a UUID suffix to the filename if there's
a collision, but the preview URL was being constructed using the original
filename instead of the actual saved filename. Now uses saved_preview_path.name.
- Remove nltk dependency from step4_assets by inlining _extract_user_id
- Add graceful error handling for step4_assets in podcast-only mode
- Fix media_utils.py to use tenant-aware get_podcast_media_read_dirs()
- Resolves avatar image 404 during scene image generation
