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_download_service.py

@@ -16,7 +16,14 @@ pytestmark = pytest.mark.asyncio
 class FakeArtistRepo:
     async def get_or_create(self, name: str) -> Artist:
         now = dt.datetime.now(dt.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,
+        )
 
 
 class FakeTrackRepo:
@@ -46,6 +53,7 @@ class FakeTrackRepo:
             metadata_status=str(kw["metadata_status"]),
             metadata_error=None,
             enriched_at=None,
+            availability="local",
             created_at=now,
             updated_at=now,
         )
@@ -140,6 +148,7 @@ def _track(source: str, source_id: str) -> Track:
         metadata_status="pending",
         metadata_error=None,
         enriched_at=None,
+        availability="local",
         created_at=now,
         updated_at=now,
     )