feature: Phase 18.1 — derive Opus 320 + seek-index sidecar at ingest

Background-job transcode (ffmpeg/libopus) after source store; pure C# Ogg
walker builds the 0.5s-bucketed granule→byte seek index + captures the
OpusHead/OpusTags setup header into a per-track sidecar in a new track-opus
vault. Best-effort, additive, regenerated on replace-audio.

DeepDrftTests/UploadDuplicateDetectionTests.cs

@@ -77,6 +77,7 @@ public class UploadDuplicateDetectionTests
 
         return new UnifiedTrackService(
             content, sqlTrackService, fileDatabase!, waveforms,
+            new NoOpOpusTranscodeQueue(),
             NullLogger<UnifiedTrackService>.Instance);
     }