perf(reco): batch id->entity hydration

radio/similar resolved ids one get_by_id at a time (N round-trips). Add
TrackRepository.get_many and use it (plus ArtistRepository.get_many) to
hydrate in a single query, preserving requested order.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/infrastructure/db/repositories/track_repository.py

@@ -46,6 +46,16 @@ class SqlAlchemyTrackRepository:
         row = await self._session.get(TrackModel, track_id)
         return _to_entity(row) if row is not None else None
 
+    async def get_many(self, ids: list[uuid.UUID]) -> list[Track]:
+        if not ids:
+            return []
+        rows = (
+            (await self._session.execute(select(TrackModel).where(TrackModel.id.in_(ids))))
+            .scalars()
+            .all()
+        )
+        return [_to_entity(r) for r in rows]
+
     async def get_by_source(self, source: str, source_id: str) -> Track | None:
         row = (
             await self._session.execute(