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.

DeepDrftPublic.Client/Services/AudioInteropService.cs

@@ -70,6 +70,36 @@ public class AudioInteropService : IAsyncDisposable
         return await InvokeJsAsync<AudioOperationResult>("DeepDrftAudio.initializeStreaming", playerId, totalStreamLength, contentType);
     }
 
+    /// <summary>
+    /// Probes whether this browser can decode Ogg Opus via <c>decodeAudioData</c> (Safari &lt; 18.4 cannot).
+    /// Phase 18 capability gate (OQ2): the player only requests Opus when this returns true, otherwise it
+    /// stays on the universal lossless path (AC7 — no listener ever gets silence over a codec gap). Probe
+    /// failures degrade to <c>false</c> (assume incapable) so an interop error can never silence playback.
+    /// </summary>
+    public async Task<bool> CanDecodeOggOpus(string playerId)
+    {
+        try
+        {
+            return await _jsRuntime.InvokeAsync<bool>("DeepDrftAudio.canDecodeOggOpus");
+        }
+        catch
+        {
+            return false;
+        }
+    }
+
+    /// <summary>
+    /// Hands the raw Opus seek/setup sidecar bytes (setup header + granule→byte seek index) to the JS player
+    /// so the next Opus stream's decoder has them BEFORE init (the 18.4 set-before-init contract). The player
+    /// parses and stashes them; <see cref="InitializeStreaming"/> applies them when it builds the Opus decoder.
+    /// Must be called before <see cref="InitializeStreaming"/> on an Opus stream. Returns the parse result —
+    /// a failure means the bytes were not a valid sidecar, and the caller falls back to lossless.
+    /// </summary>
+    public async Task<AudioOperationResult> SetOpusSidecar(string playerId, byte[] sidecarBytes)
+    {
+        return await InvokeJsAsync<AudioOperationResult>("DeepDrftAudio.setOpusSidecar", playerId, sidecarBytes);
+    }
+
     public async Task<StreamingResult> ProcessStreamingChunk(string playerId, byte[] audioChunk)
     {
         return await InvokeJsAsync<StreamingResult>("DeepDrftAudio.processStreamingChunk", playerId, audioChunk);