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/lib/auth.ts

@@ -3,6 +3,7 @@ import { prismaAdapter } from "better-auth/adapters/prisma";
 import { admin } from "better-auth/plugins";
 import { prisma } from "./prisma";
 import bcrypt from "bcryptjs";
+import { sendWelcomeEmail } from "./email";
 
 export const auth = betterAuth({
   database: prismaAdapter(prisma, {
@@ -16,6 +17,15 @@ export const auth = betterAuth({
       verify: ({ hash, password }) => bcrypt.compare(password, hash),
     },
   },
+  databaseHooks: {
+    user: {
+      create: {
+        after: async (user) => {
+          await sendWelcomeEmail(user.email, user.name);
+        },
+      },
+    },
+  },
   plugins: [
     admin({
       defaultRole: "student",