Convert database to InnoDB. This should probably be a script, containing not only lines like:

ALTER TABLE xxx ENGINE = InnoDB;

But also lines adding FOREIGN KEY constraints to all the tables now that we can actually enforce them. It looks like (most of?) these already exist in the schema but none of them will be there due to former use of MyISAM; simply taking these and converting them to ALTER TABLE statements should do.

Adding the ENGINE statements to the CREATE TABLE in the schema script would also be good.

Noted omissions of foreign keys looking at the schema:
* Sessions.UsersID - needs a foreign key, ON DELETE CASCADE as well (do users ever get deleted?)
* Packages.{Submitter,Maintainer}UID - mark NULL, DEFAULT NULL, ON DELETE SET NULL so foreign key integrity is preserved. There is no user with Id 0.
* PackageDepends - needs two foreign keys, ON DELETE CASCADE
* PackageSources - foreign key + ON DELETE CASCADE
* PackageComments - What is DelUsersID? And default probably needs to be NULL
* TU_VoteInfo, TU_Votes - needs constraints similar to what has been outlined so far