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/DTOs/ReleaseDto.cs

@@ -14,7 +14,16 @@ public class ReleaseDto : BaseModel
     public string? Genre { get; set; }
     public DateOnly? ReleaseDate { get; set; }
     public string? ImagePath { get; set; }
-    public ReleaseType ReleaseType { get; set; } = ReleaseType.Single;
+    public ReleaseMedium Medium { get; set; } = ReleaseMedium.Cut;
+
+    // Nullable: meaningful only for Cut releases. TrackConverter nulls it for Session/Mix at the
+    // mapping point. One producer enforces the rule; no consumer depends on a non-null value.
+    public ReleaseType? ReleaseType { get; set; }
+
+    // Medium-specific satellites. Populated only for the matching medium; null otherwise.
+    public SessionMetadataDto? SessionMetadata { get; set; }
+    public MixMetadataDto? MixMetadata { get; set; }
+
     public long? CreatedByUserId { get; set; }
 
     // Read-model field: count of non-deleted tracks in this release. Not on ReleaseEntity — the