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>

render-worker/scripts/still_render.py

@@ -728,9 +728,16 @@ def main():
                         continue
             except Exception:
                 pass
-        if not gpu_found:
+        if gpu_found:
+            print(f"RENDER_DEVICE_USED: engine=CYCLES device=GPU compute_type={device_type}", flush=True)
+        else:
             scene.cycles.device = 'CPU'
             print("[still_render] WARNING: GPU not found — falling back to CPU")
+            print("RENDER_DEVICE_USED: engine=CYCLES device=CPU compute_type=NONE (fallback)", flush=True)
+            import os as _os
+            if _os.environ.get("CYCLES_DEVICE", "auto").lower() == "gpu":
+                print("GPU_REQUIRED_BUT_CPU_USED: strict mode active (CYCLES_DEVICE=gpu)", flush=True)
+                sys.exit(2)
 
     # ── Render settings ──────────────────────────────────────────────────────
     scene.render.resolution_x = width