Spec 1 of 2 for the 2FA challenge rework. Moves the backup (recovery) code
off the combined challenge screen into a dedicated /two-factor-challenge/backup
route + view, behind a 'Backup-Code verwenden' button; the main screen shows
only the primary factor. A key-only user without secure context (http + bare IP)
is redirected straight to the backup view. Shared rate-limit/login logic is
extracted into a CompletesTwoFactorChallenge trait so both views hit one bucket
set. IP-vs-domain factor routing is already correct (WebauthnService::available)
and unchanged. Spec 2 (auth-failure -> fail2ban hard IP ban) is out of scope here.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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