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/domain/entities/album.py

@@ -13,5 +13,7 @@ class Album:
     year: int | None
     cover_path: str | None
     musicbrainz_id: str | None
+    source: str | None
+    source_id: str | None
     created_at: dt.datetime
     updated_at: dt.datetime