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/infrastructure/db/repositories/artist_repository.py

@@ -15,6 +15,8 @@ def _to_entity(row: ArtistModel) -> Artist:
     return Artist(
         id=row.id,
         name=row.name,
+        source=row.source,
+        source_id=row.source_id,
         created_at=row.created_at,
         updated_at=row.updated_at,
     )
@@ -35,6 +37,32 @@ class SqlAlchemyArtistRepository:
             await self._session.refresh(row)
         return _to_entity(row)
 
+    async def get_or_create_remote(self, *, name: str, source: str, source_id: str) -> Artist:
+        """Resolve an artist by ``(source, source_id)`` first (re-browse/save
+        dedup), falling back to ``name`` and gap-filling the remote ids onto an
+        existing row, else creating a new remote-bound row."""
+        row = (
+            await self._session.execute(
+                select(ArtistModel).where(
+                    ArtistModel.source == source,
+                    ArtistModel.source_id == source_id,
+                )
+            )
+        ).scalar_one_or_none()
+        if row is None:
+            row = (
+                await self._session.execute(select(ArtistModel).where(ArtistModel.name == name))
+            ).scalar_one_or_none()
+        if row is None:
+            row = ArtistModel(name=name, source=source, source_id=source_id)
+            self._session.add(row)
+        elif row.source is None and row.source_id is None:
+            row.source = source
+            row.source_id = source_id
+        await self._session.flush()
+        await self._session.refresh(row)
+        return _to_entity(row)
+
     async def get_by_id(self, artist_id: uuid.UUID) -> Artist | None:
         row = await self._session.get(ArtistModel, artist_id)
         return _to_entity(row) if row is not None else None