refactor: migrate to public Piston API with Redis rate limiter

Replace Piston with custom subprocess executor

Fix Dockerfile to build ON ghcr.io piston image to meet <1GB limit while ensuring piston actually runs

Revert to lightweight piston binary install

Update piston image to ghcr.io

Update Dockerfile, supervisord.conf, and install_runtimes.sh

Update Dockerfile, supervisord.conf, and install_runtime.sh

Add runtime, FastAPI main, and worker