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/api/routers/worker.py

@@ -16,6 +16,7 @@ from app.models.order_line import OrderLine
 from app.models.product import Product
 from app.models.user import User
 from app.models.worker_config import WorkerConfig
+from app.models.system_setting import SystemSetting
 from app.utils.auth import get_current_user, require_admin_or_pm, require_admin
 
 router = APIRouter(prefix="/worker", tags=["worker"])
@@ -456,6 +457,34 @@ async def scale_workers(
     return {"service": body.service, "count": body.count, "status": "scaling"}
 
 
+# ---------------------------------------------------------------------------
+# GPU probe
+# ---------------------------------------------------------------------------
+
+@router.post("/probe/gpu", status_code=http_status.HTTP_202_ACCEPTED)
+async def trigger_gpu_probe(current_user: User = Depends(require_admin)):
+    """Queue a GPU probe task on the render-worker."""
+    from app.tasks.gpu_tasks import probe_gpu
+    result = probe_gpu.delay()
+    return {"task_id": str(result.id), "queued": True}
+
+
+@router.get("/probe/gpu/result")
+async def get_gpu_probe_result(
+    current_user: User = Depends(require_admin),
+    db: AsyncSession = Depends(get_db),
+):
+    """Return the last GPU probe result from system_settings."""
+    import json
+    row = await db.execute(
+        select(SystemSetting).where(SystemSetting.key == "gpu_probe_last_result")
+    )
+    setting = row.scalar_one_or_none()
+    if not setting:
+        return {"status": "unknown", "message": "No probe run yet. Click Run GPU Check."}
+    return json.loads(setting.value)
+
+
 # ---------------------------------------------------------------------------
 # Render health check
 # ---------------------------------------------------------------------------