fix(render): persist OptiX BVH cache across render-worker rebuilds

Mount named volume optix-cache:/root/.nv so the OptiX ComputeCache
survives docker compose rebuild. Without this every rebuild wiped the
BVH acceleration structure, causing the first render of any complex
scene (~175 parts) to take 130–150s instead of 22s while OptiX
recompiles kernels and rebuilds the BVH from scratch.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -142,6 +142,7 @@ services:
       - ./backend:/app
       - uploads:/app/uploads
       - /opt/blender:/opt/blender:ro
+      - optix-cache:/root/.nv          # persist OptiX BVH/kernel cache across rebuilds
     depends_on:
       postgres:
         condition: service_healthy
@@ -200,3 +201,4 @@ volumes:
   pgdata:
   uploads:
   minio-data:
+  optix-cache:     # OptiX BVH/kernel pipeline cache (survives render-worker rebuilds)