db: scope DestroyUser to only delete the target user's pre-auth keys

DestroyUser called ListPreAuthKeys(tx) which returns ALL pre-auth keys
across all users, then deleted every one of them. This caused deleting
any single user to wipe out pre-auth keys for every other user.

Extract a ListPreAuthKeysByUser function (consistent with the existing
ListNodesByUser pattern) and use it in DestroyUser to scope key deletion
to the user being destroyed.

Add unit test (table-driven in TestDestroyUserErrors) and integration
test to prevent regression.

Fixes #3154

Co-authored-by: Kristoffer Dalby <kristoffer@dalby.cc>