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/NoOpOpusTranscodeQueue.cs

@@ -0,0 +1,16 @@
+using DeepDrftAPI.Services.Opus;
+
+namespace DeepDrftTests;
+
+/// <summary>
+/// Test double for <see cref="IOpusTranscodeQueue"/>. The background Opus derive is out of scope for the
+/// upload/dual-database tests — those assert SQL + source-vault behaviour, which is unchanged by Phase 18
+/// (Opus is strictly additive). Enqueuing is a no-op so the upload path under test never spins up a real
+/// transcode. Records the enqueued keys in case a test wants to assert the scheduling contract.
+/// </summary>
+public sealed class NoOpOpusTranscodeQueue : IOpusTranscodeQueue
+{
+    public List<string> Enqueued { get; } = [];
+
+    public void Enqueue(string entryKey) => Enqueued.Add(entryKey);
+}