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>

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> {