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/curator/homework/[submissionId]/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 { sendFeedbackReceivedEmail } from "@/lib/email";
 
 export async function submitFeedback(submissionId: string, text: string) {
   const session = await auth.api.getSession({ headers: await headers() });
@@ -15,6 +16,36 @@ export async function submitFeedback(submissionId: string, text: string) {
     data: { submissionId, curatorId: session.user.id, text },
   });
 
+  // Send email to student
+  const submission = await prisma.homeworkSubmission.findUnique({
+    where: { id: submissionId },
+    include: {
+      user: { select: { email: true, name: true } },
+      homework: {
+        include: {
+          lesson: {
+            select: {
+              title: true,
+              module: { select: { course: { select: { slug: true } } } },
+            },
+          },
+        },
+      },
+    },
+  });
+
+  if (submission) {
+    const { lesson } = submission.homework;
+    const lessonUrl = `${process.env.BETTER_AUTH_URL ?? "https://school.second-brain.ru"}/courses/${lesson.module.course.slug}/lessons/${submission.homework.lessonId}`;
+    await sendFeedbackReceivedEmail(
+      submission.user.email,
+      submission.user.name,
+      lesson.title,
+      text,
+      lessonUrl
+    );
+  }
+
   revalidatePath("/curator/homework");
   revalidatePath(`/curator/homework/${submissionId}`);
 }