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/SessionMetadata.cs

@@ -0,0 +1,17 @@
+using Models.Entities;
+
+namespace DeepDrftModels.Entities;
+
+// 1:1 satellite for Session-medium releases (Phase 9). One row per Session ReleaseEntity, keyed by
+// a unique ReleaseId FK (the 1:1 enforcement lives in SessionMetadataConfiguration). Carries the
+// hero-image entry key into the Image vault.
+//
+// Inherits Id, CreatedAt, UpdatedAt, IsDeleted from BaseEntity (Cerebellum.BlazorBlocks.Models).
+// BaseEntity ships the audit columns but does not declare IEntity itself, so subclasses declare it
+// explicitly to satisfy the generic constraints on Repository<>/Manager<>/etc.
+public class SessionMetadata : BaseEntity, IEntity
+{
+    public long ReleaseId { get; set; }
+    public ReleaseEntity Release { get; set; } = null!;
+    public required string HeroImageEntryKey { get; set; }
+}