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/email.ts

@@ -137,6 +137,31 @@ export async function sendHomeworkSubmittedEmail(
   }).catch((e) => console.error("[email] sendHomeworkSubmittedEmail:", e));
 }
 
+export async function sendCommentNotificationEmail(
+  to: string,
+  recipientName: string,
+  studentName: string,
+  lessonTitle: string,
+  commentText: string,
+  lessonUrl: string
+) {
+  const school = await getSchoolName();
+  const esc = (t: string) => t.replace(/&/g, "&amp;").replace(/</g, "&lt;").replace(/>/g, "&gt;");
+  const preview = commentText.length > 400 ? commentText.slice(0, 400) + "…" : commentText;
+  await getResend().emails.send({
+    from: FROM,
+    to,
+    subject: `Новый комментарий под уроком — ${lessonTitle}`,
+    html: base(`
+      <p ${p}>Привет${recipientName ? `, ${esc(recipientName)}` : ""}!</p>
+      <p ${p}><strong>${esc(studentName)}</strong> оставил комментарий под уроком <strong>«${lessonTitle}»</strong>:</p>
+      ${quote(esc(preview))}
+      <p ${pLast}>Открыть урок и ответить:</p>
+      ${btn(lessonUrl, "Открыть урок")}
+    `, school),
+  }).catch((e) => console.error("[email] sendCommentNotificationEmail:", e));
+}
+
 export async function sendFeedbackReceivedEmail(
   to: string,
   studentName: string,