feat(backend): add Artist entity and track ownership

Track.Artist was a plain string snapshot with no identity, and
UserHasTrack doubled as both "in my library" and "I own this",
conflating collection membership with administrative ownership.

Introduce Artist/TrackArtist to give artist credits their own
identity (needed so YouTube tracks with multiple credited artists,
and future renames, can be modeled), and Track.OwnerUserId to
represent ownership explicitly (local tracks only, enforced by a
check constraint). CreateTrack now provisions the uploader's Artist
row and links it via TrackArtist; DeleteTrack authorizes against
OwnerUserId instead of any UserHasTrack row.

backend/Musify.Infrastructure/Persistence/Database.cs

@@ -34,6 +34,45 @@ namespace Musify.Infrastructure.Persistence
                 .HasIndex(track => new { track.Source, track.ExternalId })
                 .IsUnique();
 
+            modelBuilder.Entity<Track>()
+                .HasOne(track => track.Owner)
+                .WithMany()
+                .HasForeignKey(track => track.OwnerUserId)
+                .OnDelete(DeleteBehavior.Restrict);
+
+            modelBuilder.Entity<Track>()
+                .ToTable(t => t.HasCheckConstraint(
+                    "CK_Tracks_OwnerUserId_Source",
+                    "(\"OwnerUserId\" IS NOT NULL) = (\"Source\" = 0)"));
+
+            modelBuilder.Entity<TrackArtist>()
+                .HasKey(trackArtist => new { trackArtist.TrackId, trackArtist.ArtistId });
+
+            modelBuilder.Entity<TrackArtist>()
+                .HasOne(trackArtist => trackArtist.Track)
+                .WithMany(track => track.TrackArtists)
+                .HasForeignKey(trackArtist => trackArtist.TrackId)
+                .OnDelete(DeleteBehavior.Cascade);
+
+            modelBuilder.Entity<TrackArtist>()
+                .HasOne(trackArtist => trackArtist.Artist)
+                .WithMany(artist => artist.TrackArtists)
+                .HasForeignKey(trackArtist => trackArtist.ArtistId)
+                .OnDelete(DeleteBehavior.Cascade);
+
+            modelBuilder.Entity<Artist>()
+                .HasIndex(artist => artist.UserId)
+                .IsUnique();
+
+            modelBuilder.Entity<Artist>()
+                .HasIndex(artist => artist.ExternalId)
+                .IsUnique();
+
+            modelBuilder.Entity<Artist>()
+                .HasIndex(artist => artist.NormalizedName)
+                .IsUnique()
+                .HasFilter("\"ExternalId\" IS NULL AND \"UserId\" IS NULL");
+
             var playListProcessing = modelBuilder.Entity<PlayListProcessingState>();
             playListProcessing.HasKey(state => state.CorrelationId);
             playListProcessing.Property(state => state.CorrelationId).ValueGeneratedNever();
@@ -44,6 +83,10 @@ namespace Musify.Infrastructure.Persistence
 
         public DbSet<Track> Tracks => Set<Track>();
 
+        public DbSet<Artist> Artists => Set<Artist>();
+
+        public DbSet<TrackArtist> TrackArtists => Set<TrackArtist>();
+
         public DbSet<PlayList> PlayLists => Set<PlayList>();
 
         public DbSet<UserHasTrack> UserHasTracks => Set<UserHasTrack>();