fix: paginate weekly digest cron to prevent serverless timeout

[Shanidhya01]

Summary

Fixes a scalability issue in the weekly digest cron job where a single unbounded Supabase query loaded all opted-in users into memory at once, causing Vercel serverless timeout failures and partial email delivery at large user counts. Users are now fetched in paginated pages of 50 and processed in parallel sub-batches of 5 via Promise.allSettled.

Closes #1852

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Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ⚡ Performance improvement

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What Changed

• src/app/api/cron/weekly-digest/route.ts — replaced the single unbounded .select() with a while (hasMore) pagination loop using Supabase .range(page * PAGE_SIZE, (page + 1) * PAGE_SIZE - 1)
• Added PAGE_SIZE = 50 constant to control DB fetch size per round-trip (distinct from the existing BATCH_SIZE = 5 which controls email concurrency within a page)
• Moved the "no users opted in" early-return to after the loop, since total count is unknown before iteration
• Renamed response fields: sentCount → emailsSent, failedCount → emailsFailed; added totalUsersProcessed to the JSON response and the completion log line
• Updated the module-level JSDoc to reflect the new two-level execution model (page → sub-batch)

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How to Test

1. Seed the users table with more than 50 rows where weekly_digest_opt_in = true and email IS NOT NULL.
2. Trigger the endpoint: curl -H "Authorization: Bearer <CRON_SECRET>" http://localhost:3000/api/cron/weekly-digest
3. Inspect the server logs — you should see multiple page-turn iterations and a final summary log.

Expected result:
Response body contains { success: true, totalUsersProcessed: N, emailsSent: N, emailsFailed: 0, skippedCount: 0, errors: [] } with totalUsersProcessed matching the seeded count. No Vercel timeout. Re-triggering within 6 days skips all users and returns skippedCount: N.

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Checklist

• Linked the related issue above
• Self-reviewed my own diff
• No unnecessary console.log, debug code, or commented-out blocks
• npm run lint passes locally
• No TypeScript errors (npm run type-check)
• Added or updated tests where applicable
• Updated documentation / comments if behavior changed

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Additional Context

Two-level batching: PAGE_SIZE = 50 and BATCH_SIZE = 5 are intentionally separate constants. PAGE_SIZE controls DB memory footprint per query; BATCH_SIZE controls Resend API concurrency. Both can be tuned independently without touching logic.

Partial-run safety: If a DB error occurs mid-pagination, the handler returns 500, but the last_digest_sent_at cooldown guard on already-sent users prevents duplicates on the next retry.

No breaking changes: Auth flow, skippedCount, errors array shape, and the message: "No users opted in" response are all preserved.

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[github-actions]

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[Priyanshu-byte-coder]

This PR has merge conflicts with the current main branch. Please rebase or merge main into your branch to resolve the conflicts, then push the updated branch. Once conflicts are resolved, this PR will be merged.

git fetch origin main
git merge origin/main
# resolve conflicts
git push

[Shanidhya01]

@Priyanshu-byte-coder Done