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.

DeepDrftAPI/Controllers/TrackController.cs

@@ -295,6 +295,15 @@ public class TrackController : ControllerBase
             {
                 var error = result.Messages.FirstOrDefault()?.Message ?? "Failed to process and store audio";
                 _logger.LogWarning("UploadTrack: UnifiedTrackService failed for {TrackName}: {Error}", trackName, error);
+
+                // A cardinality rejection is a well-formed request that violates a domain rule, so it
+                // is 409 Conflict — distinct from the 500 used for processing failure. The marker is
+                // stripped so the client sees only the human-readable detail.
+                if (error.StartsWith(UnifiedTrackService.CardinalityViolationMarker, StringComparison.Ordinal))
+                {
+                    return Conflict(error[UnifiedTrackService.CardinalityViolationMarker.Length..]);
+                }
+
                 return StatusCode(500, error);
             }