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/check_version.py

@@ -64,5 +64,62 @@ def check_version():
     print(f"Blender {version_str} OK (>= {MIN_VERSION_STR})")
 
 
+def check_gpu():
+    """Run the Blender GPU probe script and report results.
+
+    Respects CYCLES_DEVICE env var:
+      - "cpu" → skip probe entirely
+      - "gpu" → require GPU; abort startup if none found
+      - "auto" (default) → warn if no GPU found, but continue
+    """
+    cycles_device = os.environ.get("CYCLES_DEVICE", "auto").lower()
+    if cycles_device == "cpu":
+        print("[check_version] GPU check skipped (CYCLES_DEVICE=cpu)", flush=True)
+        return
+
+    blender_bin = find_blender()
+    probe_script = Path("/render-scripts/gpu_probe.py")
+    if not probe_script.exists():
+        print(
+            f"[check_version] WARNING: gpu_probe.py not found at {probe_script}",
+            flush=True,
+        )
+        return
+
+    try:
+        result = subprocess.run(
+            [blender_bin, "--background", "--python", str(probe_script)],
+            capture_output=True, text=True, timeout=45,
+        )
+        if result.returncode == 0:
+            for line in result.stdout.splitlines():
+                if "GPU_PROBE_OK" in line:
+                    print(f"[check_version] {line}", flush=True)
+                    break
+        else:
+            msg = "No GPU detected — renders will use CPU"
+            for line in result.stdout.splitlines():
+                if "GPU_PROBE_FAIL" in line:
+                    msg = line
+                    break
+            if cycles_device == "gpu":
+                print(f"[check_version] ERROR: {msg}", flush=True)
+                print(
+                    "[check_version] CYCLES_DEVICE=gpu requires GPU — aborting startup",
+                    flush=True,
+                )
+                sys.exit(1)
+            else:
+                print(
+                    f"[check_version] WARNING: {msg} (set CYCLES_DEVICE=gpu to enforce)",
+                    flush=True,
+                )
+    except subprocess.TimeoutExpired:
+        print("[check_version] WARNING: GPU probe timed out after 45s", flush=True)
+    except Exception as e:
+        print(f"[check_version] WARNING: GPU probe failed: {e}", flush=True)
+
+
 if __name__ == "__main__":
     check_version()
+    check_gpu()