Tadkira: when concert ticket sales open, almost every buyer gets an error

Tadkira is a fictional online ticketing service (www.tadkira.ma): concerts, matches, festivals. Purchases go through API Gateway (REST API tadkira-api, stage prod) then through the Lambda function tadkira-reservation, which locks the seats in DynamoDB.

This morning at 10:00, ticket sales opened for a festival's big closing concert: about 40,000 people were waiting. Since then, most buyers see “Internal error, try again”. Social media is on fire and the organizer calls every two minutes.

Three months ago, the team ran load tests on this function. You are the on-call cloud engineer, with the AWS CLI v2 (region eu-west-3).