Construct the go-webauthn verifier at the composition root and attach
it to the API when auth.panel_hostname is configured (RP id = panel
hostname, origin = https://<panel hostname>, display name Felis). When
the hostname is unset or the verifier fails to build it stays nil and
the passkey ceremony routes report 503, matching the existing
nil-when-unconfigured subsystem pattern. An admin passkey, if ever
added, is a separate relying party on the admin host and is
intentionally not wired here.
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