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_import_service.py

@@ -20,7 +20,14 @@ class FakeArtistRepo:
     async def get_or_create(self, name: str) -> Artist:
         if name not in self._by_name:
             now = dt.datetime.now(dt.UTC)
-            self._by_name[name] = Artist(id=uuid.uuid4(), name=name, created_at=now, updated_at=now)
+            self._by_name[name] = Artist(
+                id=uuid.uuid4(),
+                name=name,
+                source=None,
+                source_id=None,
+                created_at=now,
+                updated_at=now,
+            )
         return self._by_name[name]
 
 
@@ -55,6 +62,7 @@ class FakeTrackRepo:
             metadata_status=str(kw["metadata_status"]),
             metadata_error=None,
             enriched_at=None,
+            availability="local",
             created_at=now,
             updated_at=now,
         )
@@ -139,6 +147,7 @@ async def test_dedup_skips_already_imported() -> None:
         metadata_status="pending",
         metadata_error=None,
         enriched_at=None,
+        availability="local",
         created_at=now,
         updated_at=now,
     )