feat: add GET /api/auth/captcha endpoint

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

server/api/auth/captcha.get.ts

@@ -0,0 +1,6 @@
+import { generateCaptcha } from '../../utils/auth/captcha'
+
+export default defineWrappedResponseHandler(async () => {
+  const { token, svg } = generateCaptcha()
+  return R.success({ token, svg })
+})