fix(K): correct CATALOG_SCRIPT path + register AssetLibrary in models/__init__

- fix(tasks): use RENDER_SCRIPTS_DIR env var for catalog_assets.py path
  (was computing wrong path via __file__ parents → /render-worker/scripts/ which doesn't exist in container)
- fix(models): add AssetLibrary to app/models/__init__.py so alembic autogenerate discovers it
- fix(api): remove unused FileResponse import from asset_libraries.py

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/api/routers/asset_libraries.py

@@ -4,7 +4,6 @@ import shutil
 from pathlib import Path
 
 from fastapi import APIRouter, Depends, HTTPException, UploadFile, File, Form, status
-from fastapi.responses import FileResponse
 from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy import select
 from pydantic import BaseModel