feat(api,admin-ui): add optional signup email-enumeration protection

[markwylde]

Summary

• add a users.prevent_email_enumeration_on_registration setting (default false)
• add duplicate-signup anti-enumeration behavior gated by:
  • setting enabled
  • email verification enabled
  • SMTP available
• add new template signup_existing_account_notice and wire it through API + admin UI email template editor
• fall back to explicit A user with this email address already exists conflict when gating conditions are not met or notice-email sending fails
• add API model tests for duplicate-registration behavior across toggle/gating combinations

Validation

• npm run tidy
• npm run build

Notes

• this keeps the default behavior explicit (duplicate email conflict) unless admins intentionally opt into anti-enumeration mode.