Enforce per-medium track cardinality in the upload service via MediumRules

Promote the Session/Mix single-track rule from a CMS-form convention to a
domain invariant: declare cardinality as data in MediumRules, enforce it in
UnifiedTrackService before the vault write (no orphan), return 409, and read
the same rule in the batch-form collapse.

DeepDrftData/ITrackService.cs

@@ -36,6 +36,15 @@ public interface ITrackService
     Task<ResultContainer<ReleaseDto>> FindOrCreateRelease(
         string title, string artist, ReleaseDto releaseData, CancellationToken cancellationToken = default);
 
+    /// <summary>
+    /// Read-only peek for an existing release by its natural key, or null when none exists — a find
+    /// with no create side-effect. Backs the upload cardinality pre-check, which must read a release's
+    /// medium and live-track count before deciding whether to admit an upload, without creating a
+    /// release for an upload it may reject. The returned DTO carries TrackCount.
+    /// </summary>
+    Task<ResultContainer<ReleaseDto?>> GetReleaseByTitleAndArtist(
+        string title, string artist, CancellationToken cancellationToken = default);
+
     Task<ResultContainer<TrackDto>> Create(TrackDto newTrack);
     Task<ResultContainer<TrackDto>> Update(TrackDto track);
     Task<Result> Delete(long id);