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/celery_app.py

@@ -30,7 +30,7 @@ celery_app.conf.update(
     enable_utc=True,
     task_routes={
         "app.domains.pipeline.tasks.*": {"queue": "step_processing"},
-        "app.domains.rendering.tasks.*": {"queue": "thumbnail_rendering"},
+        "app.domains.rendering.tasks.*": {"queue": "asset_pipeline"},
         "app.tasks.beat_tasks.*": {"queue": "step_processing"},
         "app.tasks.ai_tasks.*": {"queue": "ai_validation"},
         # Legacy task names (shim) — keep until old queued tasks drain