I see MariaDB has an auditing module... if that isn't overkill.

Otherwise it can't be very hard to pull together a list of checksums of table structure, with their constraints/indexes (and maybe a few more things). Then run that daily/hourly/whatever, redirect to timestamped files, and make sure you notice when a diff appears.

(Postgres has a configuration setting 'log_statement' that can be set to 'ddl', which logs CREATE, ALTER, and DROP statements. Ideal, but now you have to go through the social tension of getting your client to migrate to postgresql :P )

[1] postgres runtime config - Logging what