Prevent false positives from isSeeded()

[ThatOneBro]

Currently the implementation is this:

function isSeeded(): Promise<User | undefined> {
  const systemRepo = getSystemRepo();
  return systemRepo.searchOne({ resourceType: 'User' });
}

This works most of the time but doesn't detect broken states automatically and can lead to weird behavior in tests, especially if Redis is caching resources from a previous environment but the database is fresh.