fix: persist OptiX kernel cache — 9x faster first render after restart

The OptiX cache was mounted at /root/.nv but NVIDIA writes to
/var/tmp/OptixCache_root/optix7cache.db (28MB). Fixed volume mount.

Before: first render after container restart = 181s (OptiX recompilation)
After: first render after container restart = 20s (cached kernels)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -142,7 +142,7 @@ services:
       - ./backend:/app
       - uploads:/app/uploads
       - /opt/blender:/opt/blender:ro
-      - optix-cache:/root/.nv          # persist OptiX BVH/kernel cache across rebuilds
+      - optix-cache:/var/tmp/OptixCache_root   # persist OptiX kernel cache across container restarts
     depends_on:
       postgres:
         condition: service_healthy