feat: add Discord bot integration for streak notifications

[VIDYANKSHINI]

Summary

This PR implements the Discord bot integration for streak notifications. It allows users to connect a Discord Webhook URL in their settings and select their local timezone. A background cron job checks hourly and sends a "streak at risk" notification at 8 PM in the user's local timezone if they haven't committed that day. It also sends milestone alerts and a weekly coding summary on Sundays.

Closes #1062

Type of Change

• Bug fix
• New feature
• Documentation update
• Refactor / code cleanup

Changes Made

• Added discord_webhook_url, timezone, and last_discord_notification_at columns to the users table.
• Added a "Discord Integration" section in src/app/dashboard/settings/page.tsx with a Webhook input, Timezone selector, and a "Test Notification" button.
• Created a utility src/lib/discord.ts to format and send rich Discord Embed messages.
• Created /api/user/settings/discord-test endpoint for sending test webhooks.
• Added /api/notifications/discord-sync endpoint and an hourly cron job in vercel.json to evaluate local timezones and trigger daily/weekly notifications.
• Updated src/app/api/user/settings/route.ts to handle Discord settings updates.

How to Test

Steps for the reviewer to verify this works:

1. Create a Discord Webhook URL in your test server.
2. Go to Settings, scroll down to Discord Integration.
3. Paste the Webhook URL and select your timezone, then click Save Discord Settings.
4. Click Test Notification and verify that a test embed appears in your Discord channel.
5. (Optional) Run the local environment and manually trigger the /api/notifications/discord-sync endpoint with a valid CRON_SECRET header to test streak at-risk and milestone notifications.

Checklist

• Linked issue in summary
• npm run lint passes locally
• No TypeScript errors (npm run type-check)
• Self-reviewed the diff
• Added/updated tests if applicable

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[github-actions]

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