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/TrackManager.cs

@@ -3,6 +3,7 @@ using Data.Managers;
 using DeepDrftData.Repositories;
 using DeepDrftModels.DTOs;
 using DeepDrftModels.Entities;
+using DeepDrftModels.Enums;
 using Microsoft.Extensions.Logging;
 using Models.Common;
 using NetBlocks.Models;
@@ -263,7 +264,10 @@ public class TrackManager
                     releaseEntity.Genre = release.Genre;
                     releaseEntity.ReleaseDate = release.ReleaseDate;
                     releaseEntity.ImagePath = release.ImagePath;
-                    releaseEntity.ReleaseType = release.ReleaseType;
+                    releaseEntity.Medium = release.Medium;
+                    // DTO ReleaseType is nullable (meaningful only for Cut); the entity field is not.
+                    // Default to Single when null, matching TrackConverter.Convert(ReleaseDto).
+                    releaseEntity.ReleaseType = release.ReleaseType ?? ReleaseType.Single;
                     releaseEntity.CreatedByUserId = release.CreatedByUserId;
                     await Repository.UpdateReleaseAsync(releaseEntity);
                 }