Critical Fix: Align internal port to 8000 to satisfy OpenEnv library requirements

Final fix for Docker registry failures: use stable python 3.12 and pin dependencies

Fix validator pipeline: python 3.12, grader POST, websockets

Fix Docker build: use python:3.11-slim-bookworm for stable registry resolution

fix: Align Container Port to 7860 for Hugging Face Space stability

Fix: Revert to port 8000 and remove custom /web route for proper openenv push integration

Fix: Use port 7860 for HF Spaces Docker SDK compatibility

Refactor: Move Dockerfile to repository root to guarantee validation script passes local Docker build step