Set Email

Set primary email for phone-only or incomplete users.

Used during account completion and other email collection points. The email is:

1. Set as primary (unverified) - for communications TO user
2. Verification email sent - so they can use it for forwarding

Returns 409 if email already exists on another account.

The email is mandatory for account completion but verification remains optional. get_user_by_email() checks both primary and verified emails, so an unverified primary email set here still prevents another account from claiming it.