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/Data/Configurations/ReleaseConfiguration.cs

@@ -42,6 +42,18 @@ public class ReleaseConfiguration : BaseEntityConfiguration<ReleaseEntity>
             .HasMaxLength(500)
             .HasColumnName("image_path");
 
+        // ReleaseType is meaningful ONLY when Medium == Cut. It is the Cut medium's discriminator
+        // data and lives on the base table by deliberate, named exception:
+        //   A CutMetadata satellite (mirroring SessionMetadata/MixMetadata) was considered and
+        //   rejected. ReleaseType is read on every card of the /cuts browse — the highest-traffic
+        //   read in the system. Moving it to a satellite would put a join on that hot path. So it
+        //   stays here. Future media MUST NOT copy this pattern: the default is a satellite metadata
+        //   table; this is the one allowed exception, justified solely by the /cuts read volume.
+        //
+        // The "ReleaseType only for Cut" invariant is advisory — enforced at the service layer and
+        // surfaced via the nullable ReleaseDto.ReleaseType (nulled for non-Cut at the converter).
+        // It is NOT a DB check constraint by choice, not necessity: EF supports HasCheckConstraint,
+        // but the invariant is advisory and we keep the schema free of it.
         builder.Property(e => e.ReleaseType)
             .IsRequired()
             .HasConversion<string>()   // Store as readable string, not int ordinal
@@ -49,6 +61,13 @@ public class ReleaseConfiguration : BaseEntityConfiguration<ReleaseEntity>
             .HasColumnName("release_type")
             .HasDefaultValue(ReleaseType.Single);
 
+        builder.Property(e => e.Medium)
+            .IsRequired()
+            .HasConversion<string>()   // Store as readable string, not int ordinal
+            .HasMaxLength(20)
+            .HasColumnName("medium")
+            .HasDefaultValue(ReleaseMedium.Cut);   // Existing rows migrate to Cut with no data migration.
+
         builder.Property(e => e.CreatedByUserId)
             .HasColumnName("created_by_user_id");