Wire Opus end-to-end playback + Backfill-Opus action (Phase 18.5)

Player picks Opus when the browser can decode it and a sidecar exists (else lossless), injecting the sidecar before stream init; seek reuses the same format. Adds the Backfill-Opus bulk API endpoint + CMS action.

DeepDrftTests/UploadDuplicateDetectionTests.cs

@@ -4,6 +4,7 @@ using Data.Managers;
 using DeepDrftAPI.Services;
 using DeepDrftContent;
 using DeepDrftContent.Processors;
+using DeepDrftContent.Processors.Opus;
 using DeepDrftData;
 using DeepDrftData.Data;
 using DeepDrftData.Repositories;
@@ -74,10 +75,13 @@ public class UploadDuplicateDetectionTests
         var waveforms = new WaveformProfileService(
             fileDatabase!, new AudioProcessor(), new RmsLoudnessAlgorithm(),
             Options.Create(new WaveformProfileOptions()), NullLogger<WaveformProfileService>.Instance);
+        var resolver = new TrackFormatResolver(
+            fileDatabase!, content, NullLogger<TrackFormatResolver>.Instance);
 
         return new UnifiedTrackService(
             content, sqlTrackService, fileDatabase!, waveforms,
             new NoOpOpusTranscodeQueue(),
+            resolver,
             NullLogger<UnifiedTrackService>.Instance);
     }