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/TrackConverter.cs

@@ -1,5 +1,6 @@
 using DeepDrftModels.DTOs;
 using DeepDrftModels.Entities;
+using DeepDrftModels.Enums;
 using Models.Converters;
 
 namespace DeepDrftData;
@@ -25,7 +26,24 @@ public class TrackConverter : IEntityToModelConverter<TrackEntity, TrackDto>
         Genre = entity.Genre,
         ReleaseDate = entity.ReleaseDate,
         ImagePath = entity.ImagePath,
-        ReleaseType = entity.ReleaseType,
+        Medium = entity.Medium,
+        // ReleaseType is meaningful only for Cut; null it for Session/Mix at the mapping point so no
+        // consumer mistakes a stale studio-format value for a live/mix release.
+        ReleaseType = entity.Medium == ReleaseMedium.Cut ? entity.ReleaseType : (ReleaseType?)null,
+        SessionMetadata = entity.SessionMetadata is null
+            ? null
+            : new SessionMetadataDto
+            {
+                ReleaseId = entity.SessionMetadata.ReleaseId,
+                HeroImageEntryKey = entity.SessionMetadata.HeroImageEntryKey
+            },
+        MixMetadata = entity.MixMetadata is null
+            ? null
+            : new MixMetadataDto
+            {
+                ReleaseId = entity.MixMetadata.ReleaseId,
+                WaveformEntryKey = entity.MixMetadata.WaveformEntryKey
+            },
         CreatedByUserId = entity.CreatedByUserId
     };
 
@@ -39,7 +57,10 @@ public class TrackConverter : IEntityToModelConverter<TrackEntity, TrackDto>
         Genre = dto.Genre,
         ReleaseDate = dto.ReleaseDate,
         ImagePath = dto.ImagePath,
-        ReleaseType = dto.ReleaseType,
+        Medium = dto.Medium,
+        // Entity ReleaseType is non-nullable; default back to Single when the DTO nulled it for a
+        // non-Cut release. Primarily a write-path reconstruction concern.
+        ReleaseType = dto.ReleaseType ?? ReleaseType.Single,
         CreatedByUserId = dto.CreatedByUserId
     };