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/models/__init__.py

@@ -11,7 +11,7 @@ from app.domains.orders.models import Order, OrderItem, OrderLine
 from app.domains.notifications.models import AuditLog
 from app.domains.billing.models import PricingTier
 from app.domains.rendering.models import OutputType, RenderTemplate, ProductRenderPosition, WorkflowDefinition, WorkflowRun, WorkflowNodeResult
-from app.domains.materials.models import Material, MaterialAlias
+from app.domains.materials.models import Material, MaterialAlias, AssetLibrary
 from app.domains.media.models import MediaAsset, MediaAssetType
 
 # Also re-export SystemSetting (no domain assigned — stays as-is)
@@ -21,5 +21,5 @@ __all__ = [
     "Tenant", "User", "Template", "CadFile", "Product", "Order", "OrderItem", "OrderLine",
     "AuditLog", "PricingTier", "OutputType", "RenderTemplate", "ProductRenderPosition",
     "WorkflowDefinition", "WorkflowRun", "WorkflowNodeResult",
-    "Material", "MaterialAlias", "MediaAsset", "MediaAssetType", "SystemSetting",
+    "Material", "MaterialAlias", "AssetLibrary", "MediaAsset", "MediaAssetType", "SystemSetting",
 ]