Adds nullable storage fields + availability column on tracks, remote
source/source_id identity on albums/artists, TrackRepository.materialize()
and get_or_create_remote() repos — groundwork for on-demand YTM library
(placeholders saved without audio, materialized in-place on first play).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Senko-san