feat(fc2b): importer enqueues tag_and_embed + ml-worker model self-heal

import_media_file now enqueues tag_and_embed alongside generate_thumbnail
after a successful import. scripts/download_models.py snapshots Camie +
SigLIP into /models, idempotent (skips when present). The ml-worker
entrypoint runs it before starting the Celery worker so a fresh /models
volume self-heals on first boot. Downloader tests are pure-logic (no
network in CI).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

entrypoint.sh

@@ -37,6 +37,8 @@ case "$ROLE" in
     ;;
 
   ml-worker)
+    echo "[entrypoint] Ensuring ML models present in /models..."
+    python -m backend.app.scripts.download_models
     echo "[entrypoint] Starting ML Celery worker (ml queue)"
     exec celery -A backend.app.celery_app:celery worker \
       --loglevel=info \