When the user selects a LoRA on a flake and no cover has been picked
yet, fetch the LoRA's sibling image (same basename, .png/.jpg/.webp/...)
from models/loras/ and use it as the flake's default cover.
Triggers from all three LoRA selection paths (file browser, dropdown
change, dropdown blur) but only for the first LoRA on the flake, and
only when no cover file has been chosen by the user. Adds a server
endpoint /flakes/lora_sibling_image mirroring the existing checkpoint
sibling endpoint, with a shared helper.