Phase 9 Wave 1: add ReleaseMedium discriminator + Session/Mix metadata

Add ReleaseMedium enum (Cut/Session/Mix) and two 1:1 satellite entities
(SessionMetadata, MixMetadata) with EF configs and an additive migration.
ReleaseDto.ReleaseType is now nullable, nulled for non-Cut at the converter.
Existing releases default to Cut via column default; no data migration.

DeepDrftData/Data/DeepDrftContext.cs

@@ -12,6 +12,8 @@ public class DeepDrftContext : DbContext
 
     public DbSet<TrackEntity> Tracks { get; set; }
     public DbSet<ReleaseEntity> Releases { get; set; }
+    public DbSet<SessionMetadata> SessionMetadata { get; set; }
+    public DbSet<MixMetadata> MixMetadata { get; set; }
 
     protected override void OnModelCreating(ModelBuilder modelBuilder)
     {
@@ -19,5 +21,7 @@ public class DeepDrftContext : DbContext
 
         modelBuilder.ApplyConfiguration(new TrackConfiguration());
         modelBuilder.ApplyConfiguration(new ReleaseConfiguration());
+        modelBuilder.ApplyConfiguration(new SessionMetadataConfiguration());
+        modelBuilder.ApplyConfiguration(new MixMetadataConfiguration());
     }
 }