fix: revert dual queue to single GPU — light worker caused 2x regression

Root cause: render-worker and render-worker-light shared the same GPU,
causing contention. Complex TRB renders went from 17s → 36s (2x slower).

Changes:
- Thumbnails back to asset_pipeline queue (not asset_pipeline_light)
- Dispatch routing always uses asset_pipeline (no queue splitting)
- render-worker-light gated behind "multi-gpu" profile — only starts with:
  docker compose --profile multi-gpu up -d
- For single-GPU setups: all rendering is sequential on one worker

The dual queue approach is correct for multi-GPU machines where each
worker gets its own GPU. On single-GPU, serial execution is faster
than concurrent GPU contention.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -157,6 +157,7 @@ services:
               capabilities: [gpu, compute, utility, graphics]
 
   render-worker-light:
+    profiles: ["multi-gpu"]   # Only start with: docker compose --profile multi-gpu up -d
     build:
       context: .
       dockerfile: render-worker/Dockerfile