fix(backend): stop loading a user's entire listening history into memory

GetListeningHistory used AsEnumerable() before DistinctBy/Take, which
forced EF Core to materialize every history row for the user just to
keep the last 15 distinct tracks. Now groups by track and takes the
15 most recently played track ids in SQL, then fetches only those.

backend/Musify.Application/Tracks/GetListeningHistory.cs

@@ -1,4 +1,5 @@
 using Mediator;
+using Microsoft.EntityFrameworkCore;
 using Musify.Application.Contracts;
 using Musify.Application.Tracks.Responses;
 
@@ -12,18 +13,24 @@ namespace Musify.Application.Tracks
     {
         public const int ListSize = 15;
 
-        public ValueTask<IEnumerable<TrackApplicationResponse>> Handle(GetListeningHistoryQuery query, CancellationToken cancellationToken)
+        public async ValueTask<IEnumerable<TrackApplicationResponse>> Handle(GetListeningHistoryQuery query, CancellationToken cancellationToken)
         {
-            var listeningHistory = database.ListeningHistories
+            var recentTrackIds = await database.ListeningHistories
                 .Where(l => l.UserId == query.UserId)
-                .OrderByDescending(l => l.ListenedAt)
-                .Select(t => new { t.Track, ListensCount = t.Track.ListeningHistories.Count })
-                .AsEnumerable()
-                .DistinctBy(t => t.Track.Id)
+                .GroupBy(l => l.TrackId)
+                .OrderByDescending(g => g.Max(l => l.ListenedAt))
+                .Select(g => g.Key)
                 .Take(ListSize)
-                .Select(t => TrackApplicationResponse.FromEntity(t.Track, t.ListensCount));
+                .ToListAsync(cancellationToken);
 
-            return ValueTask.FromResult(listeningHistory);
+            var tracksById = await database.Tracks
+                .Where(t => recentTrackIds.Contains(t.Id))
+                .Select(t => new { t.Id, Track = t, ListensCount = t.ListeningHistories.Count })
+                .ToDictionaryAsync(t => t.Id, cancellationToken);
+
+            return recentTrackIds
+                .Select(id => tracksById[id])
+                .Select(t => TrackApplicationResponse.FromEntity(t.Track, t.ListensCount));
         }
     }
 }