refactor: rename thumbnail_rendering queue to asset_pipeline

The queue handles far more than thumbnails: OCC tessellation, USD master
generation, GLB production, order line renders, and workflow renders.
asset_pipeline better reflects its role as the render-worker's primary queue.

Updated all references in: task decorators, celery_app.py, beat_tasks.py,
docker-compose.yml worker command, worker.py MONITORED_QUEUES, admin.py,
CLAUDE.md, LEARNINGS.md, Dockerfile, helpTexts.ts, test files,
and all .claude/commands/*.md skill files.

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

backend/tests/conftest.py

@@ -230,7 +230,6 @@ def mock_celery_tasks(monkeypatch):
         "app.domains.materials.tasks.refresh_asset_library_catalog",
         "app.tasks.step_tasks.process_step_file",
         "app.tasks.step_tasks.render_step_thumbnail",
-        "app.tasks.step_tasks.generate_stl_cache",
         "app.domains.imports.tasks.validate_excel_import",
         "app.domains.rendering.tasks.render_still_task",
         "app.domains.rendering.tasks.render_turntable_task",