[FEAT] : add Timezone-Aware Weekly Digest (Aesthetics & UX)

[hrshjswniii]

Summary

Updates the weekly digest email cron job to deliver digests at Monday 09:00 AM in the user's local timezone instead of a single global UTC slot. It shifts the cron run schedule from once-weekly to hourly, matches users dynamically, and fixes a non-deterministic pagination bug in the users table database query.

Closes #2982

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Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that changes existing behavior)
• 📝 Documentation update
• ♻️ Refactor / code cleanup (no functional change)
• ⚡ Performance improvement
• 🔒 Security fix
• 🧪 Tests only

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What Changed

• [vercel.json]:
  • Modified the schedule parameter for /api/cron/weekly-digest from once-weekly (0 9 * * 1) to hourly (0 * * * *).
• [src/app/api/cron/weekly-digest/route.ts]:
  • Added isUserLocalTimeTargetRange helper using Intl.DateTimeFormat to resolve local hours and weekdays for users.
  • Implemented currentWeekLabelInTimezone to compute week labels matching the local Monday date. This resolves skew errors in eastern timezones (e.g. Sydney, Tokyo) where Sunday UTC is already Monday local.
  • Fixed a pagination bug by inserting an explicit .order("id") instruction before the .range() block to ensure deterministic database query paging.
  • Integrated the timezone checks in the execution loop and tracked hourly skips with a new skippedTimezoneCount property.

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How to Test

1. Opt-in a test user to receive weekly digests and set their timezone setting (e.g., America/New_York or Asia/Kolkata).
2. Run the cron endpoint locally (supplying the Authorization header).
3. If the current time matches Monday 9 AM in the test user's timezone, verify the email is sent successfully.
4. If the current time does not match Monday 9 AM, verify that the console outputs skipped_timezone for that user and that the return payload increments skippedTimezoneCount.
5. Verify that the email's subject line displays the correct date of Monday in the user's timezone.

Expected result: Users only receive digests at Monday 9 AM local time, and week label dates are calculated correctly based on local offsets.

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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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Accessibility (UI changes only)

N/A - Backend / configuration change.

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Additional Context

The idempotency guard cooldown (DIGEST_COOLDOWN_MS = 6 days) is preserved, guaranteeing that even if a user switches timezones or crons overlap, they will never receive duplicate digest emails in the same week.

[github-actions]

GSSoC Label Checklist 🏷️

@Priyanshu-byte-coder — please apply the appropriate labels before merging:

Difficulty (pick one):

• level:beginner — 20 pts
• level:intermediate — 35 pts
• level:advanced — 55 pts
• level:critical — 80 pts

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• quality:clean — ×1.2 multiplier
• quality:exceptional — ×1.5 multiplier

Validation (required to score):

• gssoc:approved — counts for points
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Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
Points formula: (difficulty × quality_multiplier) + type_bonus

[hrshjswniii]

Hiii @Priyanshu-byte-coder , do review the issue implemented and feel free to merge the Pr !!

[Priyanshu-byte-coder]

New feature/UI PRs are on hold during the code-quality sprint — see discussion #2651. We'll revisit this after the sprint; meanwhile tests, refactors, and bug-fix PRs are very welcome.