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

package.json

@@ -21,6 +21,7 @@
     "lucide-react": "^1.14.0",
     "nanoid": "^5.1.11",
     "next": "16.2.6",
+    "next-auth": "^4.24.14",
     "postgres": "^3.4.9",
     "react": "19.2.4",
     "react-dom": "19.2.4",