Use cs as the only public subject for coding retrieval (LeetCode-backed)

[wellingtonmapise]

Summary

This PR makes cs the only public API subject for coding-problem retrieval.

Internally, retrieval still uses the existing LeetCode artifacts (LEETCODE_*), but clients now send subject: "cs".

Changes

• Updated src/routes.py:
  • subject="cs" now maps to the LeetCode retrieval pipeline.
  • subject="leetcode" is now rejected with a 400 and message:
    • "subject 'leetcode' is not supported. Use 'cs'."
  • Subject validation now allows only:
    • "math" and "cs"
• Updated README.md:
  • Replaced LeetCode subject examples with subject: "cs".
  • Updated supported subject docs to "math" and "cs".
  • Updated curl example to use "cs".

Why

• Keeps frontend/API contract simple and stable.
• Uses domain-facing naming (cs) instead of data-source naming (leetcode).
• Preserves existing artifact pipeline without renaming files/config yet.

Testing

• Positive test:
  • POST /api/search with {"subject":"cs","query":"two pointers array","top_k":5}
  • returns ranked coding-problem results.
• Negative test:
  • POST /api/search with {"subject":"leetcode",...}
  • returns 400 with unsupported-subject message.

[Tomtom-debug]

since we are catching all other cases at the end no need for the first if.