SES receives Gusto payroll emails at payroll@int.seahaven.com, stores
to S3, Lambda parses and posts a combined Slack notification (employee
payroll + contractor payments in one message) after a 10-minute SQS
batching window.
Removes Aurora Serverless, notifyPayroll Lambda, and @aws-sdk/client-rds-data.
Adds mailparser, SQS delay queue, and processPayrollEmail Lambda.
