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/alembic/versions/054_worker_configs.py

@@ -26,7 +26,7 @@ def upgrade() -> None:
         INSERT INTO worker_configs (queue_name, max_concurrency, min_concurrency, enabled)
         VALUES
             ('step_processing', 8, 2, true),
-            ('thumbnail_rendering', 1, 1, true),
+            ('asset_pipeline', 1, 1, true),
             ('ai_validation', 4, 1, true)
         ON CONFLICT DO NOTHING
     """)