Mixer should user faker.safe_email by default

Seems like a sane default to me to use `safe_email` over `email` just in case anyone ever plugs mixer into a test suite that actually sends emails without realising.

Start by locating where Mixer chooses Faker's email provider and where its fixture-generation tests cover email values; the issue names `email` and `safe_email` but no files or tests. Change the default to `safe_email`, then confirm the existing test coverage verifies the generated value uses the safe provider.

Written by the indexing model from the issue text.