Configure app for Hugging Face Gradio SDK deployment

Phase 9 prep: Add modal_app.py, update CI/CD for Modal.com, clean up HF Spaces files, update README

Phase 9: Add Gradio Space app (gradio_app.py), HF Spaces README frontmatter, updated requirements.txt + CI/CD workflow

Phase 9: Production Dockerfile, pinned requirements.txt, and GitHub Actions CI/CD deploy workflow for HuggingFace Spaces