Every route in py/api/images.py and py/api/prompts.py now has a success
path and an error path test: save/update/delete/rate/tag/category/bulk
endpoints, image list/link/unlink/delete/metadata/scan endpoints, the
thumbnail SSE stream, clear-thumbnails, and the video thumbnail chain
(fake cv2 success/failure, ffmpeg absent or failing, PIL placeholder).
Combined statement+branch coverage: images.py 97.7%, prompts.py 97.4%.