[FEATURE] Implement email notification system for saved searches

[ascender1729]

Description

Allow users to subscribe to email notifications for new issues matching their saved searches.

Why Needed

• Stay updated on new issues without checking manually
• Catch good issues before others
• Proactive contribution opportunities

Implementation Details

• Email subscription form
• Store preferences in D1 database
• Scheduled Worker to check for new issues
• Email sending via Mailgun/SendGrid/Resend
• Unsubscribe functionality

Architecture

1. User saves search with email notification enabled
2. Preference stored in D1 with search criteria
3. Cron Worker runs periodically (hourly/daily)
4. Checks for new issues matching criteria
5. Sends digest email if new issues found

Files to Create/Modify

• workers/notifications.ts (cron handler)
• workers/email.ts (email sending)
• src/components/shared/EmailSubscribe.svelte
• Database schema for subscriptions

Acceptance Criteria

• Users can subscribe to notifications
• Email sent when new issues match
• Unsubscribe link in emails
• Rate limiting on emails
• Preferences editable

Time Estimates

• With AI: 145 minutes
• Without AI: 960 minutes