refactor(models): merge downloaders into one, driven by models.yml

Replace the per-model downloader services with a single downloader
that reads file/url pairs from models.yml. Downloads now go to a
.part file first, so an interrupted run can't leave a truncated
model that later runs skip.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scripts/update.sh

@@ -175,7 +175,6 @@ docker compose build --pull
 
 echo "==> ensuring models are downloaded (skips already-present files)"
 docker compose --profile tools run --rm downloader
-docker compose --profile tools run --rm downloader-draft
 
 echo "==> bringing up omniroute"
 docker compose up -d --wait omniroute