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/MixMetadata.cs

@@ -0,0 +1,17 @@
+using Models.Entities;
+
+namespace DeepDrftModels.Entities;
+
+// 1:1 satellite for Mix-medium releases (Phase 9). One row per Mix ReleaseEntity, keyed by a unique
+// ReleaseId FK (the 1:1 enforcement lives in MixMetadataConfiguration). Carries the entry key for
+// the preprocessed high-resolution waveform datum.
+//
+// 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 MixMetadata : BaseEntity, IEntity
+{
+    public long ReleaseId { get; set; }
+    public ReleaseEntity Release { get; set; } = null!;
+    public required string WaveformEntryKey { get; set; }
+}

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; }
 }

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; }
+}