List Passkeys

The caller's own passkeys. Never anybody else's, and never key material.

Deliberately not behind require_passkey_support, and not filtered to the current relying-party ID. A deployment that has changed or lost that ID still holds the rows registered under the old one, and refusing to list them would leave somebody looking at an empty page with no way to clean up and no hint as to why. Each row carries is_usable instead, so an orphan is visible, explained, and deletable.