feat(02-01): install next-auth@4, configure CredentialsProvider auth

- Add next-auth@4 dependency (stable v4, not beta v5)
- Create src/lib/auth.ts with CredentialsProvider validating ADMIN_EMAIL/ADMIN_PASSWORD env vars
- Create src/app/api/auth/[...nextauth]/route.ts catch-all handler (GET + POST)
- JWT session strategy — stateless, no DB users table
- Custom sign-in page set to /admin/login
- Add NEXTAUTH_URL, NEXTAUTH_SECRET, ADMIN_EMAIL, ADMIN_PASSWORD to .env.local

src/app/api/auth/[...nextauth]/route.ts

@@ -0,0 +1,6 @@
+import NextAuth from "next-auth";
+import { authOptions } from "@/lib/auth";
+
+const handler = NextAuth(authOptions);
+
+export { handler as GET, handler as POST };