Fix JSON parse error on empty responses: handle empty body in API client and return message from saveSmtpConfig

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

admin/src/lib/api.ts

@@ -24,7 +24,8 @@ async function request<T>(path: string, options?: RequestInit): Promise<T> {
     throw new ApiError(res.status, body.message || 'Error de servidor');
   }
 
-  return res.json();
+  const text = await res.text();
+  return text ? JSON.parse(text) : ({} as T);
 }
 
 export const api = {

backend/src/auth/email-otp.service.ts

@@ -46,6 +46,7 @@ export class EmailOtpService {
       create: { key: 'smtp_config', value: config as any },
       update: { value: config as any },
     });
+    return { message: 'Configuración SMTP guardada' };
   }
 
   async sendOtp(email: string): Promise<void> {