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Add an `OAUTH_EMAIL_CLAIM` variable to override the default "email" claim value.

this commit adds an optional environment variable named `WEBUI_AUTH_TRUSTED_NAME_HEADER`, which sets the user's name to the contents of that header. this only happens if the user is just being created, just like how the trusted e-mail header works.

if the environment variable or header is not present, we fall back to the original behavior which is to re-use the user e-mail address.
Co-Authored-By: Nikita Borzykh <sample@fastmail.com>