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/domains/materials/tasks.py

@@ -3,6 +3,7 @@ from __future__ import annotations
 
 import json
 import logging
+import os
 import subprocess
 import uuid
 from pathlib import Path
@@ -11,7 +12,8 @@ from celery import shared_task
 
 logger = logging.getLogger(__name__)
 
-CATALOG_SCRIPT = Path(__file__).parent.parent.parent.parent.parent / "render-worker" / "scripts" / "catalog_assets.py"
+# Scripts are copied to /render-scripts/ in the render-worker container (RENDER_SCRIPTS_DIR env var)
+CATALOG_SCRIPT = Path(os.environ.get("RENDER_SCRIPTS_DIR", "/render-scripts")) / "catalog_assets.py"
 
 
 @shared_task(
@@ -48,7 +50,6 @@ def refresh_asset_library_catalog(self, asset_library_id: str) -> None:
             return
 
         # Determine Blender binary
-        import os
         blender_bin = os.environ.get("BLENDER_BIN", "blender")
 
         result = subprocess.run(