fix(relay): add MissingHeader doc comment; restore correct #[allow(dead_code)] scope
Minor 3 from final code review: clarify why DpopTokenEndpointError::MissingHeader carries
#[allow(dead_code)] — the handler pre-checks for a missing DPoP header before calling
validate_dpop_for_token_endpoint, so this variant is never constructed in practice but
exists for exhaustive match completeness.
Also includes CLAUDE.md and db/CLAUDE.md freshness updates from the project librarian step.