fix(K): asset library catalog refresh — fix task binding and model imports

- fix(tasks): @shared_task → @celery_app.task so task uses configured redis://redis broker
  (@shared_task without explicit app bound to default app → localhost:6379 → Connection refused)
- fix(tasks): import app.models before DB access so SQLAlchemy can resolve Material.creator → User
  relationship (was: InvalidRequestError: 'User' failed to locate a name)

Verified: catalog refresh now finds and stores 35 materials from uploaded .blend file.

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

backend/app/domains/materials/tasks.py

@@ -8,7 +8,7 @@ import subprocess
 import uuid
 from pathlib import Path
 
-from celery import shared_task
+from app.tasks.celery_app import celery_app
 
 logger = logging.getLogger(__name__)
 
@@ -16,7 +16,7 @@ logger = logging.getLogger(__name__)
 CATALOG_SCRIPT = Path(os.environ.get("RENDER_SCRIPTS_DIR", "/render-scripts")) / "catalog_assets.py"
 
 
-@shared_task(
+@celery_app.task(
     name="app.domains.materials.tasks.refresh_asset_library_catalog",
     queue="thumbnail_rendering",
     bind=True,
@@ -30,6 +30,7 @@ def refresh_asset_library_catalog(self, asset_library_id: str) -> None:
     """
     from sqlalchemy import create_engine
     from sqlalchemy.orm import Session
+    import app.models  # noqa: F401 — registers all SQLAlchemy models so relationships resolve
     from app.domains.materials.models import AssetLibrary
     from app.config import settings