feat: add GPU support via Dockerfile.gpu and docker-compose.gpu.yml

- Restore original Dockerfile and docker-compose.yml (no breaking changes)
- Add Dockerfile.gpu with NVIDIA CUDA 12.1 base image
- Add docker-compose.gpu.yml as compose override for GPU deployment
- Update GPU deployment documentation with both CPU and GPU instructions

CPU users: docker compose up -d (unchanged)
GPU users: docker compose -f docker-compose.yml -f docker-compose.gpu.yml up -d

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