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/SessionMetadataDto.cs

@@ -0,0 +1,10 @@
+namespace DeepDrftModels.DTOs;
+
+// Mirror of SessionMetadata (Phase 9). No `required` members — BlazorBlocks's Manager<> generic
+// constraint requires `new()`, which does not compose with required members. TrackConverter assigns
+// every field on the round-trip, so an empty default is never observable.
+public class SessionMetadataDto
+{
+    public long ReleaseId { get; set; }
+    public string HeroImageEntryKey { get; set; } = string.Empty;
+}