feat(library): lazy materialization foundation for remote tracks (§Phase1)

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>

app/api/rest/serializers.py

@@ -80,8 +80,8 @@ def song_dict(
         "albumId": encode_album(track.album_id) if track.album_id is not None else None,
         "artistId": encode_artist(track.artist_id),
         "coverArt": cover,
-        "size": track.file_size,
-        "contentType": content_type_for(track.file_format),
+        "size": track.file_size or 0,
+        "contentType": content_type_for(track.file_format or ""),
         "suffix": track.file_format,
         "duration": track.duration_seconds,
         "year": track.year,