Add Vercel cron job for cache building

[neubig]

This PR adds a Vercel cron job configuration that runs npm run build:cache every 12 hours.

Changes:

• Added vercel.json with cron job configuration
• Created /api/cron.js endpoint to handle the cron job

Note: A CRON_SECRET environment variable needs to be added in Vercel project settings.

[vercel]

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