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/blender_render.py

@@ -718,9 +718,15 @@ if engine != "eevee":   # covers both explicit Cycles and EEVEE-fallback
         # Re-ensure preferences are set (engine activation may have reset them)
         _activate_gpu()
         print(f"[blender_render] Cycles GPU ({gpu_type_found}), samples={samples}", flush=True)
+        print(f"RENDER_DEVICE_USED: engine=CYCLES device=GPU compute_type={gpu_type_found}", flush=True)
     else:
         scene.cycles.device = 'CPU'
         print(f"[blender_render] WARNING: GPU not found — falling back to CPU, samples={samples}", flush=True)
+        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)
 
     scene.cycles.samples       = samples
     scene.cycles.use_denoising = True

render-worker/scripts/gpu_probe.py

@@ -0,0 +1,43 @@
+"""Blender Python script: GPU compute device probe.
+
+Run via:
+  blender --background --python gpu_probe.py
+
+Exit codes:
+  0 — GPU found, prints GPU_PROBE_OK line
+  1 — No GPU found or error, prints GPU_PROBE_FAIL line
+"""
+import sys
+
+
+def main():
+    try:
+        import bpy
+
+        cprefs = bpy.context.preferences.addons['cycles'].preferences
+        for device_type in ('OPTIX', 'CUDA', 'HIP', 'ONEAPI'):
+            try:
+                cprefs.compute_device_type = device_type
+                cprefs.get_devices()
+                gpu_devices = [d for d in cprefs.devices if d.type != 'CPU']
+                if gpu_devices:
+                    device_names = [(d.name, d.type) for d in gpu_devices]
+                    print(
+                        f"GPU_PROBE_OK: device_type={device_type} devices={device_names}",
+                        flush=True,
+                    )
+                    sys.exit(0)
+            except Exception as e:
+                print(f"GPU_PROBE: {device_type} not available: {e}", flush=True)
+
+        print("GPU_PROBE_FAIL: no GPU compute device found", flush=True)
+        sys.exit(1)
+
+    except SystemExit:
+        raise
+    except Exception as e:
+        print(f"GPU_PROBE_FAIL: exception during probe: {e}", flush=True)
+        sys.exit(1)
+
+
+main()

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

render-worker/scripts/turntable_render.py

@@ -682,9 +682,16 @@ 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("[turntable_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