feat(gpu): GPU health check + RENDER_DEVICE_USED token + strict mode

- gpu_probe.py: Blender script that probes OPTIX/CUDA/HIP/ONEAPI and
  exits 1 on no GPU — used at startup + on-demand from Admin UI
- blender_render.py, still_render.py, turntable_render.py: emit
  RENDER_DEVICE_USED: engine=CYCLES device=GPU|CPU compute_type=...
  after GPU activation; exit 2 when CYCLES_DEVICE=gpu and CPU fallback
- render_blender.py: parse RENDER_DEVICE_USED token into render_log
  (device_used, compute_type, gpu_fallback); handle exit code 2 as
  explicit GPU strict-mode failure
- check_version.py: check_gpu() runs gpu_probe.py at container startup;
  CYCLES_DEVICE=gpu aborts startup if no GPU found
- docker-compose.yml: CYCLES_DEVICE=${CYCLES_DEVICE:-auto} env var
- gpu_tasks.py: probe_gpu Celery task on thumbnail_rendering queue;
  saves result to system_settings.gpu_probe_last_result; beat every 30min
- worker.py: POST /probe/gpu (trigger) + GET /probe/gpu/result (last result)

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

backend/app/tasks/celery_app.py

@@ -18,6 +18,7 @@ celery_app = Celery(
         "app.domains.products.tasks",
         "app.domains.imports.tasks",
         "app.domains.materials.tasks",
+        "app.tasks.gpu_tasks",
     ],
 )
 
@@ -56,5 +57,9 @@ celery_app.conf.update(
             "task": "app.tasks.beat_tasks.apply_worker_concurrency",
             "schedule": 300.0,  # every 5 minutes
         },
+        "probe-gpu-every-30m": {
+            "task": "app.tasks.gpu_tasks.probe_gpu",
+            "schedule": 1800.0,  # every 30 minutes
+        },
     },
 )