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>

tests/test_real_file_enrichment.py

@@ -149,6 +149,7 @@ def _pending_track() -> Track:
         metadata_status="pending",
         metadata_error=None,
         enriched_at=None,
+        availability="local",
         created_at=now,
         updated_at=now,
     )
@@ -180,7 +181,14 @@ class _FakeArtistRepo:
     async def get_or_create(self, name: str) -> Artist:
         self.created.append(name)
         now = datetime.now(UTC)
-        return Artist(id=uuid.uuid4(), name=name, created_at=now, updated_at=now)
+        return Artist(
+            id=uuid.uuid4(),
+            name=name,
+            source=None,
+            source_id=None,
+            created_at=now,
+            updated_at=now,
+        )
 
 
 @dataclass
@@ -199,6 +207,8 @@ class _FakeAlbumRepo:
             year=year,
             cover_path=None,
             musicbrainz_id=musicbrainz_id,
+            source=None,
+            source_id=None,
             created_at=now,
             updated_at=now,
         )