frontend: archive-or-deprecate Streamlit app with explicit migration docs

[ayushkrtiwari]

Goal

Since Streamlit is now legacy, add a clear deprecation/archive strategy so users know where to go and how to restore functionality if needed. README already points to legacy Streamlit. :contentReference[oaicite:2]{index=2}

Tasks

• Add legacy/streamlit/README.md describing how to run & why it's archived.
• Add a top-level README section "Streamlit (legacy) — status: archived" linking to migration plan.
• Add a migration checklist: pages to port, data endpoints needed.
• Optionally, add a CI job that tests the Streamlit entrypoint if you intend to keep it runnable.

Acceptance criteria

• Users have clear guidance and can run or ignore legacy Streamlit without confusion.