Background GitHub stars sync with Vercel Cron

[lovincyrus]

Problem

GitHub stars sync is fully synchronous — the POST handler blocks until all GitHub API pages are fetched and inserted before responding. A user with 5,000 stars triggers ~50 sequential API calls, which can take 10-30s. On Vercel hobby tier (10s timeout), this can fail silently.

There's also no automatic re-syncing — users must manually visit Settings and click Sync each time.

Proposed Solution

waitUntil + Vercel Cron (zero new dependencies, free on hobby tier)

Manual sync (UX improvement)

• Return { message: "Sync started" } immediately using Next.js waitUntil() to decouple the response from the work
• The sync continues running in the background after the response is sent

Scheduled sync (new)

• Add a GET /api/cron/sync-github-stars route triggered daily by Vercel Cron
• Protect the route with CRON_SECRET header verification
• Iterates over all users with a configured GitHubStarsSync record and syncs each
• ETag caching means unchanged syncs cost 1 API call (~100ms), so daily runs are cheap

Cost

• Free on Vercel hobby tier (2 cron jobs included, 1/day minimum)
• waitUntil is built into Next.js
• No external services or additional API keys

Notes

• Existing ETag + skipDuplicates logic already handles idempotency
• GITHUB_TOKEN env var (optional) increases rate limit from 60 to 5,000 req/hr
• If timeout becomes a problem for very large star counts (3,000+), chunked pagination can be added later