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/TrackManager.cs

@@ -201,6 +201,25 @@ public class TrackManager
         }
     }
 
+    public async Task<ResultContainer<ReleaseDto?>> GetReleaseByTitleAndArtist(
+        string title, string artist, CancellationToken cancellationToken = default)
+    {
+        try
+        {
+            var existing = await Repository.GetReleaseByTitleAndArtistAsync(title, artist, cancellationToken);
+            if (existing is null)
+                return ResultContainer<ReleaseDto?>.CreatePassResult(null);
+
+            var dto = TrackConverter.Convert(existing);
+            dto.TrackCount = await Repository.CountLiveTracksByReleaseAsync(existing.Id, cancellationToken);
+            return ResultContainer<ReleaseDto?>.CreatePassResult(dto);
+        }
+        catch (Exception e)
+        {
+            return ResultContainer<ReleaseDto?>.CreateFailResult(e.Message);
+        }
+    }
+
     public async Task<ResultContainer<List<GenreSummaryDto>>> GetDistinctGenres(CancellationToken cancellationToken = default)
     {
         try