Implement email notifications for issue assignments and mentions #20

Problem

When someone assigns you an issue or @mentions you in a comment, there is no notification outside of the in-app inbox. Users who are not actively in OpenGrove will miss critical updates.

Expected

Email digests and real-time email alerts configurable per-user in notification preferences.

Acceptance Criteria

• Email sent when: assigned to issue, mentioned in comment, issue status changed on your assigned issues
• User preference: real-time vs daily digest vs off
• Unsubscribe link in every email
• Uses Clerk email or Convex scheduled functions for delivery
• Free tier: daily digest only. Pro: real-time available