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/admin/courses/[courseId]/actions.ts

@@ -5,6 +5,7 @@ import { headers } from "next/headers";
 import { auth } from "@/lib/auth";
 import { revalidatePath } from "next/cache";
 import { redirect } from "next/navigation";
+import { sendCourseAccessEmail } from "@/lib/email";
 
 async function requireAdmin() {
   const session = await auth.api.getSession({ headers: await headers() });
@@ -70,6 +71,16 @@ export async function grantAccess(
       note: note || null,
     },
   });
+
+  // Send email notification
+  const [user, course] = await Promise.all([
+    prisma.user.findUnique({ where: { id: userId }, select: { email: true, name: true } }),
+    prisma.course.findUnique({ where: { id: courseId }, select: { title: true } }),
+  ]);
+  if (user && course) {
+    await sendCourseAccessEmail(user.email, user.name, course.title);
+  }
+
   revalidatePath(`/admin/courses/${courseId}`);
 }