Search results now report whether a hit is already saved (in_library,
track_id, availability). New RemoteLibraryService backs POST
/tracks/remote (idempotent placeholder save) and POST
/tracks/{id}/materialize (on-demand fetch via a new materialize_track
arq task, reusing in-flight jobs).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Senko-san