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/app/tasks/beat_tasks.py

@@ -73,7 +73,7 @@ def broadcast_queue_status() -> None:
         r = sync_redis.from_url(settings.redis_url, decode_responses=True)
         depths = {
             "step_processing": r.llen("step_processing"),
-            "thumbnail_rendering": r.llen("thumbnail_rendering"),
+            "asset_pipeline": r.llen("asset_pipeline"),
         }
         event = {"type": "queue_update", "depths": depths}
         r.publish("__broadcast__", json.dumps(event))