feat: multi-GPU queue routing for legacy still renders (M2)

dispatch_order_line_render now routes still renders (non-animation,
non-cinematic) to a secondary GPU queue when MULTI_GPU_LIGHT_RENDER_QUEUE
is configured and that queue has active Celery workers.

- config.py: multi_gpu_light_render_queue setting (default "" = disabled)
- render_order_line.py: implement the routing stub — load output_type via
  selectinload, check render_settings.animation + .cinematic flags, call
  _inspect_active_worker_queues (reused from workflow_graph_runtime) with
  0.5s timeout to check if the light queue is live

No behaviour change when MULTI_GPU_LIGHT_RENDER_QUEUE is not set.
Enable by setting it to "asset_pipeline_light" and adding a
render-worker-light service with concurrency=1 to docker-compose.

docs: learnings erfasst — multi-GPU queue routing M2

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

backend/app/config.py

@@ -75,7 +75,14 @@ class Settings(BaseSettings):
 
     # Redis / Celery
     redis_url: str = "redis://localhost:6379/0"
+    # Queue for shadow-mode workflow renders (second GPU worker).
     workflow_shadow_render_queue: str = "asset_pipeline_light"
+    # When non-empty AND that queue has active workers, still renders from the
+    # legacy dispatch path are routed here instead of asset_pipeline, enabling
+    # concurrent still rendering on multi-GPU setups.
+    # Set MULTI_GPU_LIGHT_RENDER_QUEUE=asset_pipeline_light in docker-compose
+    # for the render-worker-light service and restart the workers.
+    multi_gpu_light_render_queue: str = ""
 
     @model_validator(mode="after")
     def normalize_runtime_hosts(self) -> "Settings":