Add caching mechanism with Vercel deployment support

[neubig]

This PR adds a caching mechanism to reduce GitHub API calls:

1. Cache Building:

   • Added build:cache script to create cache of GitHub API responses
   • Cache stored in .cache/github-activities.json
   • Cache includes timestamp and is valid for 24 hours

2. Environment Variables:

   • Added VITE_USE_CACHE to control caching behavior
   • Added CRON_SECRET for securing cache rebuild endpoint

3. Vercel Integration:

   • Cache built during deployment (part of build process)
   • Cache rebuilt every 6 hours via Vercel Cron Jobs
   • Added /api/rebuild-cache endpoint for manual rebuilds

4. Testing:

   • Added tests for cache building script
   • Added tests for cache usage in GitHub service
   • Added tests for rebuild-cache API endpoint
   • Added tests for error handling

[vercel]

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