feat: recovery codes #398

Why

Recovery today is admin-assisted device replacement only. A user who loses their only passkey on a small team has nowhere to go. Fear of lockout is the main objection to passwordless.

What

• Generate a set of single-use codes, store them hashed, show them once
• Sign in with a code, then force a new passkey enrollment
• Regenerate (invalidates the old set), with step-up
• Audit events for generate, use, regenerate; notify the user on use
• System config to require codes at enrollment

Done when

A user can recover with a code, every step is audited, and codes are never stored in plain text.