Adds the /Items universal query endpoint, dispatching by IncludeItemTypes
over albums/artists/songs/genres with ParentId, SearchTerm, Filters=IsFavorite,
SortBy/SortOrder and StartIndex/Limit support, plus GET /Items/{itemId} and
/Users/{userId}/Items/Latest. Reuses server/subsonic/filter builders (by
artist/album/starred) instead of hand-rolled squirrel filters, and resolves
sort keys per item type since each repository maps sort names to different
real/aliased columns.
Also adds the missing CountAll to tests.MockArtistRepo, exposed by this task
(the mock embedded a nil ArtistRepository for it and would panic on use).