Fix Resend lazy init to avoid build-time API key error

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

src/lib/email.ts

@@ -1,6 +1,8 @@
 import { Resend } from "resend";
 
-const resend = new Resend(process.env.RESEND_API_KEY);
+function getResend() {
+  return new Resend(process.env.RESEND_API_KEY);
+}
 const FROM = process.env.EMAIL_FROM ?? "noreply@mailsend.second-brain.ru";
 const BASE_URL = process.env.BETTER_AUTH_URL ?? "https://school.second-brain.ru";
 
@@ -36,7 +38,7 @@ function base(content: string) {
 // ── Email senders ─────────────────────────────────────────────────────────────
 
 export async function sendCourseAccessEmail(to: string, name: string, courseTitle: string) {
-  await resend.emails.send({
+  await getResend().emails.send({
     from: FROM,
     to,
     subject: `Вам открыт доступ к курсу «${courseTitle}»`,
@@ -56,7 +58,7 @@ export async function sendHomeworkSubmittedEmail(
   lessonTitle: string,
   submissionId: string
 ) {
-  await resend.emails.send({
+  await getResend().emails.send({
     from: FROM,
     to,
     subject: `Новая работа на проверку — ${lessonTitle}`,
@@ -76,7 +78,7 @@ export async function sendFeedbackReceivedEmail(
   feedbackText: string,
   lessonUrl: string
 ) {
-  await resend.emails.send({
+  await getResend().emails.send({
     from: FROM,
     to,
     subject: `Получен фидбек по уроку «${lessonTitle}»`,
@@ -93,7 +95,7 @@ export async function sendFeedbackReceivedEmail(
 }
 
 export async function sendWelcomeEmail(to: string, name: string) {
-  await resend.emails.send({
+  await getResend().emails.send({
     from: FROM,
     to,
     subject: "Добро пожаловать в Second Brain",