Add email notification on new lesson comment

Notify admins/curators (or configured notificationEmails) when a student
leaves a comment under a lesson. New setting notifyOnComment (default on) +
toggle in admin settings. Fires only for student comments, not admin/curator
replies; comment text is HTML-escaped in the email.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/lib/settings.ts

@@ -14,6 +14,7 @@ export const SETTINGS_DEFAULTS = {
   // Notifications
   notificationEmails: "", // newline-separated list
   notifyOnHomework: "true",
+  notifyOnComment: "true",
   notifyOnRegistration: "true",
   notifyStudentOnFeedback: "true",