feat: add weekly email digest with AI productivity summary

[Subhra-Nandi]

[FEAT] Add weekly email digest with AI productivity summary

Closes #937

What this PR does

Implements the weekly email digest feature. Opted-in users receive a Monday 9am UTC email with an AI-generated productivity summary, current streak, top repo, PR stats, and a trend badge.

Changes

File	Description
supabase/migrations/20260529000000_add_digest_settings.sql	Adds digest_unsubscribe_token to users table
src/lib/digest.ts	AI summary generator + email HTML builder
src/app/api/settings/digest/route.ts	GET/POST opt-in preference API
src/app/api/cron/weekly-digest/route.ts	Enhanced with AI summary + unsubscribe token
src/components/DigestOptIn.tsx	Toggle switch component
src/app/settings/page.tsx	New /settings page
src/app/unsubscribe/page.tsx	Token-based one-click unsubscribe
src/components/DashboardHeader.tsx	Gear icon linking to /settings
.env.example	Documents new env vars

vercel.json already had the cron entry — no changes needed.

Acceptance Criteria

• User can opt in/out in Settings
• Cron triggers every Monday at 9am UTC
• Email compatible with Gmail, Outlook, Apple Mail
• Unsubscribe link in every email
• Unsubscribe works without login

Screenshots

Required before deploying

1. Run supabase/migrations/20260529000000_add_digest_settings.sql in Supabase SQL editor
2. Add env vars in Vercel: RESEND_API_KEY, RESEND_FROM_EMAIL, GEMINI_API_KEY, CRON_SECRET

[vercel]

@Subhra-Nandi is attempting to deploy a commit to the PRIYANSHU DOSHI's projects Team on Vercel.

A member of the Team first needs to authorize it.

[github-actions]

GSSoC Label Checklist 🏷️

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[Priyanshu-byte-coder]

This PR has merge conflicts with main. Please rebase your branch against the latest main and force-push to resolve. Run: git fetch upstream && git rebase upstream/main

[Priyanshu-byte-coder]

This PR has developed merge conflicts after recent merges to main. Please rebase onto the latest main branch, resolve all conflicts, and force-push. Once the PR is conflict-free it will be reviewed for merging.