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.

DeepDrftModels/Entities/ReleaseEntity.cs

@@ -18,6 +18,13 @@ public class ReleaseEntity : BaseEntity, IEntity
     public DateOnly? ReleaseDate { get; set; }
     public string? ImagePath { get; set; }
     public ReleaseType ReleaseType { get; set; } = ReleaseType.Single;
+    public ReleaseMedium Medium { get; set; } = ReleaseMedium.Cut;
     public long? CreatedByUserId { get; set; }
     public ICollection<TrackEntity> Tracks { get; set; } = new List<TrackEntity>();
+
+    // 1:1 satellites selected by Medium. Null unless this release is the matching medium —
+    // Session releases carry SessionMetadata, Mix releases carry MixMetadata, Cut releases carry
+    // neither (ReleaseType on this table is their discriminator data).
+    public SessionMetadata? SessionMetadata { get; set; }
+    public MixMetadata? MixMetadata { get; set; }
 }