feat: enable item search

[zhangzhenghao]

Summary

• index repository names and descriptions through [recommend.search]
• upgrade PyGorse to 0.5.1 for SearchItems support
• adapt existing recommendation endpoints to PyGorse 0.5 scored responses while preserving their string-ID response shape
• have the browser extension pass the current repository description to get_neighbors_v2, and use it to search for neighbors when the repository is missing from Gorse

Verification

• python -m py_compile app.py
• node --check extension/content.js
• node --check extension/background.js
• parsed config.toml and verified the search columns
• verified the PyGorse 0.5.1 search_items signature
• audited all affected PyGorse call sites
• verified the extension description selector against a live GitHub repository page
• docker compose config --quiet