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/Enums/ReleaseMedium.cs

@@ -0,0 +1,16 @@
+namespace DeepDrftModels.Enums;
+
+/// <summary>
+/// The medium of a release — the Phase 9 discriminator that selects which metadata shape applies.
+/// </summary>
+public enum ReleaseMedium
+{
+    /// <summary>Studio recording. Uses <see cref="ReleaseType"/> (Single/EP/Album). The default.</summary>
+    Cut,
+
+    /// <summary>Single live track plus a hero image. Detail in <c>SessionMetadata</c>.</summary>
+    Session,
+
+    /// <summary>Single long track plus a preprocessed high-resolution waveform datum. Detail in <c>MixMetadata</c>.</summary>
+    Mix
+}