Jellify's Discover tab calls GET /Items/Latest and got a 404: we only routed the
/Users/{userId}/Items/Latest form, which real Jellyfin marks [Obsolete] and hides
from its OpenAPI spec, so SDK-generated clients never see it. Add the current
route alongside the legacy one, both served by the same handler.
getLatest also ignored ParentId, so browsing a library or an artist returned the
newest albums across everything the user can see. It now scopes to the library
when ParentId names one, and filters to that artist's albums otherwise, which
also makes a stale id return nothing instead of silently widening back to the
full library set. A malformed ParentId 404s, matching /Items and the contract
decodeFilterParam documents.
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