Add email notifications via Resend

- src/lib/email.ts: HTML templates for 4 email types (Second Brain design)
- Welcome email on user registration (Better Auth databaseHooks)
- Course access email when admin grants enrollment
- Homework submitted email to all admins/curators (first submission only)
- Feedback received email to student with feedback text and lesson link
- Update TECHNICAL.md: Resend domain, from-address, email vars, Stage 3 summary

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/(student)/courses/[slug]/lessons/[lessonId]/homework-actions.ts

@@ -4,6 +4,7 @@ import { prisma } from "@/lib/prisma";
 import { auth } from "@/lib/auth";
 import { headers } from "next/headers";
 import { revalidatePath } from "next/cache";
+import { sendHomeworkSubmittedEmail } from "@/lib/email";
 
 interface HomeworkFile {
   name: string;
@@ -31,15 +32,37 @@ export async function submitHomework(
     throw new Error("Работа уже проверена");
   }
 
+  let submissionId: string;
   if (existing) {
-    await prisma.homeworkSubmission.update({
+    const updated = await prisma.homeworkSubmission.update({
       where: { id: existing.id },
       data: { text, files: files as object[], submittedAt: new Date() },
     });
+    submissionId = updated.id;
   } else {
-    await prisma.homeworkSubmission.create({
+    const created = await prisma.homeworkSubmission.create({
       data: { homeworkId, userId: session.user.id, text, files: files as object[] },
     });
+    submissionId = created.id;
+
+    // Notify admins/curators on first submission only
+    const [lesson, admins] = await Promise.all([
+      prisma.homework.findUnique({
+        where: { id: homeworkId },
+        include: { lesson: { select: { title: true } } },
+      }),
+      prisma.user.findMany({
+        where: { role: { in: ["admin", "curator"] } },
+        select: { email: true, name: true },
+      }),
+    ]);
+    if (lesson) {
+      await Promise.all(
+        admins.map((a) =>
+          sendHomeworkSubmittedEmail(a.email, a.name, session.user.name, lesson.lesson.title, submissionId)
+        )
+      );
+    }
   }
 
   revalidatePath(`/courses/${slug}/lessons/${lessonId}`);