feat: recommend server-generated mixes from Musify and YouTube Music

Mixes are their own concept, not playlists: they live in their own
Mixes/MixItems tables, cannot be edited and never show up among the
user's playlists. Each item is either a Musify track or a YouTube Music
reference, so YouTube songs stream directly instead of being downloaded.

A daily Hangfire job seeds the mixes from the listening history: one
discovery mix, one daily mix and one per top artist.

The per-user YouTube filler query was already in the working tree and
rides along here because it shares files with this change.

backend/Musify.Infrastructure/Persistence/Database.cs

@@ -137,6 +137,25 @@ namespace Musify.Infrastructure.Persistence
                 .HasIndex(albumTrack => new { albumTrack.AlbumId, albumTrack.TrackId })
                 .IsUnique();
 
+            modelBuilder.Entity<Mix>()
+                .HasIndex(mix => new { mix.UserId, mix.Position });
+
+            modelBuilder.Entity<MixItem>()
+                .HasOne(item => item.Mix)
+                .WithMany(mix => mix.Items)
+                .HasForeignKey(item => item.MixId)
+                .OnDelete(DeleteBehavior.Cascade);
+
+            modelBuilder.Entity<MixItem>()
+                .HasOne(item => item.Track)
+                .WithMany()
+                .HasForeignKey(item => item.TrackId)
+                .IsRequired(false)
+                .OnDelete(DeleteBehavior.Cascade);
+
+            modelBuilder.Entity<MixItem>()
+                .HasIndex(item => new { item.MixId, item.Position });
+
             var playListProcessing = modelBuilder.Entity<PlayListProcessingState>();
             playListProcessing.HasKey(state => state.CorrelationId);
             playListProcessing.Property(state => state.CorrelationId).ValueGeneratedNever();
@@ -167,6 +186,10 @@ namespace Musify.Infrastructure.Persistence
 
         public DbSet<PlayListHasTrack> PlayListHasTracks => Set<PlayListHasTrack>();
 
+        public DbSet<Mix> Mixes => Set<Mix>();
+
+        public DbSet<MixItem> MixItems => Set<MixItem>();
+
         public DbSet<Upload> Uploads => Set<Upload>();
 
         public DbSet<UploadIntent> UploadIntents => Set<UploadIntent>();