feat: implement caching for GitHub API responses

[Ai-chan-0411]

Summary

Closes #33

Adds in-memory caching for GitHub GraphQL API responses to reduce redundant API calls and improve response times.

Changes

• lib/cache.ts — Generic cache with TTL-based expiration (default 5 min). Entries auto-expire on read.
• lib/github.ts — fetchGitHubUserData checks cache before calling GitHub API. Cache key is the lowercased username.
• app/api/compare/route.ts — Adds Cache-Control: public, s-maxage=300, stale-while-revalidate=60 header for CDN/browser caching.

How it works

1. First request for a username → hits GitHub API, stores result in cache
2. Subsequent requests within 5 minutes → served from cache (no API call)
3. After TTL expires → next request fetches fresh data

Benefits

• Reduces GitHub API rate limit consumption
• Faster responses for repeated comparisons
• Zero external dependencies (in-memory Map)

Test plan

• Compare same usernames twice — second request should be noticeably faster
• Wait 5+ minutes and compare again — should fetch fresh data
• Check response headers include Cache-Control

[vercel]

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[github-actions]

Thank you for the pull request! ✅

A maintainer will review this soon. Please be patient while we take a look. 🙌

[Ai-chan-0411]

Hi @O2sa! Just checking in — this PR implements caching for GitHub API responses (closes #33). The implementation uses an in-memory cache with configurable TTL and automatic cleanup. Ready for review whenever you get a chance! 🙏

[Ai-chan-0411]

Closing this PR as there has been no maintainer feedback for several weeks. Thank you for the opportunity to contribute! Feel free to reopen if you would like these changes.