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/application/recommendation_service.py

@@ -82,8 +82,8 @@ class RecommendationService:
         if self._recommender.is_available():
             ids = await self._recommender.similar_artist_ids(artist_id, limit=limit)
             if ids is not None:
-                found = [a for i in ids if (a := await self._artists.get_by_id(i))]
-                return REASON_ML, found
+                by_id = {a.id: a for a in await self._artists.get_many(ids)}
+                return REASON_ML, [by_id[i] for i in ids if i in by_id]
 
         found = await self._artists.list_similar(artist_id=artist_id, limit=limit)
         return REASON_SIMILAR, found
@@ -183,5 +183,7 @@ class RecommendationService:
         return None, REASON_DISCOVER
 
     async def _hydrate_tracks(self, ids: list[uuid.UUID]) -> list[Track]:
-        """Resolve ids → tracks preserving order, skipping any that vanished."""
-        return [t for i in ids if (t := await self._tracks.get_by_id(i))]
+        """Resolve ids → tracks preserving order, skipping any that vanished.
+        One batched query rather than N per-id round-trips."""
+        by_id = {t.id: t for t in await self._tracks.get_many(ids)}
+        return [by_id[i] for i in ids if i in by_id]